1. ¿Te has asegurado de codificar los valores de los parámetros en caso de que contengan caracteres especiales como espacios en blanco, puntos, etc?:

HttpUtility.UrlEncode(usuario);
HttpUtility.UrlEncode(contraseña);

2. ¿Has comprobado que le estás pasando las cabeceras correctas?.

3. De todas formas podrías necesitar más que eso, prueba a obtener la Cookie de visitante en la página principal, y luego a loguearte usando dicha cookie,
escribí este snippet al que se le puede dar un uso más o menos genérico que podría servir para tu situación, le hice unas pequeñas modificaciones para adaptarlo a tus necesidades, solo modifica los valores de la propiedades 'UrlMain', 'UrlLogin', 'UrlLoginQueryFormat', comprueba las cabeceras que asigno en 'RequestHeadersPostLogin' sean correctas y no falten más, y por último llama al método 'CheckLogin' para evaluar el login.

VB.Net:

 
    ''' <summary>
    ''' Gets the main url.
    ''' </summary>
    ''' <value>The main url.</value>
    Public ReadOnly Property UrlMain As String
        Get
            Return "http://www.cgwallpapers.com/"
        End Get
    End Property
 
    ''' <summary>
    ''' Gets the login url.
    ''' </summary>
    ''' <value>The login url.</value>
    Public ReadOnly Property UrlLogin As String
        Get
            Return "http://www.cgwallpapers.com/login.php"
        End Get
    End Property
 
    ''' <summary>
    ''' Gets the login query string format.
    ''' </summary>
    ''' <value>The login query string format.</value>
    Public ReadOnly Property UrlLoginQueryFormat As String
        Get
            Return "usuario={0}&contrasena={1}"
        End Get
    End Property
 
    ''' <summary>
    ''' Gets the headers for a Login POST request.
    ''' </summary>
    ''' <value>The headers for a Login POST request.</value>
    Public ReadOnly Property RequestHeadersPostLogin As WebHeaderCollection
        Get
 
            Dim headers As New WebHeaderCollection
            With headers
                .Add("Accept-Language", "en-us,en;q=0.5")
                .Add("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7")
                .Add("Keep-Alive", "99999")
            End With
            Return headers
 
        End Get
    End Property
 
    ''' <summary>
    ''' Determines whether the user is logged in the site.
    ''' </summary>
    Private isLogged As Boolean
 
    ''' <summary>
    ''' Gets the cookie container.
    ''' </summary>
    ''' <value>The cookie container.</value>
    Public ReadOnly Property CookieCollection As CookieCollection
        Get
            Return Me.cookieCollection1
        End Get
    End Property
    ''' <summary>
    ''' The cookie container.
    ''' </summary>
    Private cookieCollection1 As CookieCollection
 
    ''' <summary>
    ''' Defines the query data for a LoginPost request.
    ''' </summary>
    Private NotInheritable Class LoginQueryData
 
        ''' <summary>
        ''' Gets the Usuario field.
        ''' </summary>
        ''' <value>The Usuario field.</value>
        Public Property Usuario As String
 
        ''' <summary>
        ''' Gets or sets the Conteasena field.
        ''' </summary>
        ''' <value>The Conteasena field.</value>
        Public Property Contrasena As String
 
    End Class
 
    ''' <summary>
    ''' Gets a formatted <see cref="String"/> representation of a <see cref="LoginQueryData"/> object.
    ''' </summary>
    ''' <param name="loginQueryData">The <see cref="LoginQueryData"/> object that contains the login query fields.</param>
    ''' <returns>A formatted <see cref="String"/> representation of a <see cref="LoginQueryData"/> object.</returns>
    Private Function GetLoginQueryString(ByVal loginQueryData As LoginQueryData) As String
 
        Return String.Format(Me.UrlLoginQueryFormat,
                             loginQueryData.Usuario,
                             loginQueryData.Contrasena)
 
    End Function
 
    ''' <summary>
    ''' Sets the cookie container.
    ''' </summary>
    ''' <param name="url">The url.</param>
    ''' <param name="cookieCollection">The cookie collection.</param>
    ''' <returns>CookieContainer.</returns>
    Private Function SetCookieContainer(ByVal url As String,
                                        ByVal cookieCollection As CookieCollection) As CookieContainer
 
        Dim cookieContainer As New CookieContainer
        Dim refDate As Date
 
        For Each oldCookie As Cookie In cookieCollection
 
            If Not DateTime.TryParse(oldCookie.Value, refDate) Then
 
                Dim newCookie As New Cookie
                With newCookie
                    .Name = oldCookie.Name
                    .Value = oldCookie.Value
                    .Domain = New Uri(url).Host
                    .Secure = False
                End With
 
                cookieContainer.Add(newCookie)
 
            End If
 
        Next oldCookie
 
        Return cookieContainer
 
    End Function
 
    ''' <summary>
    ''' Converts cookie string to global cookie collection object.
    ''' </summary>
    ''' <param name="cookie">The cookie string.</param>
    ''' <param name="cookieCollection">The cookie collection.</param>
    Private Sub SaveCookies(ByVal cookie As String,
                            ByRef cookieCollection As CookieCollection)
 
        Dim cookieStrings() As String = cookie.Trim.
                                               Replace("path=/,", String.Empty).
                                               Replace("path=/", String.Empty).
                                               Split({";"c}, StringSplitOptions.RemoveEmptyEntries)
 
        cookieCollection = New CookieCollection
 
        For Each cookieString As String In cookieStrings
 
            If Not String.IsNullOrEmpty(cookieString.Trim) Then
 
                cookieCollection.Add(New Cookie(name:=cookieString.Trim.Split("="c)(0),
                                                value:=cookieString.Trim.Split("="c)(1)))
 
            End If
 
        Next cookieString
 
    End Sub
 
    ''' <summary>
    ''' Convert cookie container object to global cookie collection object.
    ''' </summary>
    ''' <param name="cookieContainer">The cookie container.</param>
    ''' <param name="cookieCollection">The cookie collection.</param>
    ''' <param name="url">The url.</param>
    Private Sub SaveCookies(ByVal cookieContainer As CookieContainer,
                            ByRef cookieCollection As CookieCollection,
                            ByVal url As String)
 
        cookieCollection = New CookieCollection
 
        For Each cookie As Cookie In cookieContainer.GetCookies(New Uri(url))
 
            cookieCollection.Add(cookie)
 
        Next cookie
 
    End Sub
 
    ''' <param name="url">The url.</param>
    ''' <param name="cookieCollection">The cookie collection.</param>
    ''' <returns><c>true</c> if successfull, <c>false</c> otherwise.</returns>
    Private Function GetMethod(ByVal url As String,
                               ByRef cookieCollection As CookieCollection) As Boolean
 
        Debug.WriteLine("[+] GetMethod function started.")
 
        Dim request As HttpWebRequest = Nothing
        Dim response As HttpWebResponse = Nothing
        Dim sr As StreamReader = Nothing
        Dim result As Boolean = False
 
        Try
            Debug.WriteLine("[+] Attempting to perform a request with:")
            Debug.WriteLine(String.Format("Method: {0}", "GET"))
            Debug.WriteLine(String.Format("Headers: {0}", String.Join(Environment.NewLine, Me.RequestHeadersPostLogin)))
 
            request = DirectCast(HttpWebRequest.Create(url), HttpWebRequest)
            With request
                .Method = "GET"
                .Headers = Me.RequestHeadersPostLogin
                .Accept = "text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5"
                .UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0"
                .AllowAutoRedirect = False
                .KeepAlive = True
            End With
            Debug.WriteLine("[-] Request done.")
 
            ' Get the server response.
            Debug.WriteLine("[+] Getting server response...")
            response = DirectCast(request.GetResponse, HttpWebResponse)
            Debug.WriteLine("[-] Getting server response done.")
 
            If request.HaveResponse Then
 
                ' Save the cookie info.
                Debug.WriteLine("[+] Saving cookies...")
                Me.SaveCookies(response.Headers("Set-Cookie"), cookieCollection)
                Debug.WriteLine("[-] Saving cookies done.")
 
                ' Get the server response.
                Debug.WriteLine("[+] Getting server response...")
                response = DirectCast(request.GetResponse, HttpWebResponse)
                Debug.WriteLine("[-] Getting server response done.")
 
                Debug.WriteLine("[+] Reading server response...")
                sr = New StreamReader(response.GetResponseStream)
                Using sr
                    ' Read the response from the server, but we do not save it.
                    sr.ReadToEnd()
                End Using
                result = True
                Debug.WriteLine("[-] Reading server response done.")
 
            Else ' No response received from server.
                Throw New Exception(String.Format("No response received from server with url: {0}", url))
                result = False
 
            End If
 
        Catch ex As Exception
            Throw
            result = False
 
        Finally
            If sr IsNot Nothing Then
                sr.Dispose()
            End If
            If response IsNot Nothing Then
                response.Close()
            End If
 
        End Try
 
        Debug.WriteLine("[-] GetMethod function finished.")
        Debug.WriteLine("[i] Returning result value...")
        Return result
 
    End Function
 
    ''' <param name="loginData">The login post data.</param>
    ''' <param name="cookieCollection">The cookie collection.</param>
    ''' <returns><c>true</c> if successfull, <c>false</c> otherwise.</returns>
    Private Function PostLoginMethod(ByVal loginData As LoginQueryData,
                                     ByRef cookieCollection As CookieCollection) As Boolean
 
        Debug.WriteLine("[+] PostLoginMethod function started.")
 
        Dim request As HttpWebRequest = Nothing
        Dim response As HttpWebResponse = Nothing
        Dim sw As StreamWriter = Nothing
        Dim initialCookieCount As Integer = 0
        Dim postData As String
        Dim result As Boolean = False
 
        Try
            Debug.WriteLine("[+] Attempting to perform a login request with:")
            Debug.WriteLine(String.Format("Method: {0}", "POST"))
            Debug.WriteLine(String.Format("Referer: {0}", Me.UrlMain))
            Debug.WriteLine(String.Format("Accept: {0}", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"))
            Debug.WriteLine(String.Format("ContentType: {0}", "application/x-www-form-urlencoded"))
            Debug.WriteLine(String.Format("Headers: {0}", String.Join(Environment.NewLine, Me.RequestHeadersPostLogin)))
 
            request = DirectCast(HttpWebRequest.Create(Me.UrlLogin), HttpWebRequest)
            With request
                .Method = "POST"
                .Headers = Me.RequestHeadersPostLogin
                .Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
                .ContentType = "application/x-www-form-urlencoded"
                .UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0"
                .Referer = Me.UrlMain
            End With
            Debug.WriteLine("[-] Request done.")
 
            Debug.WriteLine("[+] Passing request cookie info...")
            If cookieCollection IsNot Nothing Then ' Pass cookie info from the login page.
                request.CookieContainer = Me.SetCookieContainer(Me.UrlLogin, cookieCollection)
            End If
            Debug.WriteLine("[-] Passing request cookie info done.")
 
            ' Set the post data.
            Debug.WriteLine("[+] Setting post data with:")
            Debug.WriteLine(String.Format("Usuario: {0}", loginData.Usuario))
            Debug.WriteLine(String.Format("Contrasena: {0}", loginData.Contrasena))
            postData = Me.GetLoginQueryString(loginData)
 
            sw = New StreamWriter(request.GetRequestStream)
            Using sw
                sw.Write(postData) ' Post the data to the server.
            End Using
            Debug.WriteLine("[-] Setting post data done.")
 
            ' Get the server response.
            Debug.WriteLine("[+] Getting server response...")
            initialCookieCount = request.CookieContainer.Count
            response = DirectCast(request.GetResponse, HttpWebResponse)
            Debug.WriteLine("[-] Getting server response done.")
 
            If request.CookieContainer.Count > initialCookieCount Then ' Login successful.
                result = True
 
            Else ' Login unsuccessful.
                result = False
 
            End If
 
            Debug.WriteLine(String.Format("[i] Login response result is: {0}",
                                          If(result, "Successful (True)",
                                                     "Unsuccessful (False)")))
 
            If result Then ' Save new login cookies.
                Debug.WriteLine("[+] Saving new login cookies...")
                Me.SaveCookies(request.CookieContainer, cookieCollection, Me.UrlMain)
                Debug.WriteLine("[-] Saving new login cookies done.")
            End If
 
        Catch ex As Exception
            Throw
 
        Finally
            If sw IsNot Nothing Then
                sw.Dispose()
            End If
            If response IsNot Nothing Then
                response.Close()
            End If
 
        End Try
 
        Debug.WriteLine("[-] PostLoginMethod function finished.")
        Debug.WriteLine("[i] Returning result value...")
        Me.isLogged = result
        Return result
 
    End Function
 
    ''' <summary>
    ''' Determines whether the account can log in CGWallpapers site.
    ''' </summary>
    ''' <returns><c>true</c> if the account can log in CGWallpapers site, <c>false</c> otherwise.</returns>
    Public Function CheckLogin(ByVal username As String,
                               ByVal password As String) As Boolean
 
        If Me.GetMethod(Me.UrlMain, Me.cookieCollection1) Then
 
            Dim loginQueryData As New LoginQueryData With
                  {
                      .Usuario = HttpUtility.UrlEncode(username),
                      .Contrasena = HttpUtility.UrlEncode(password)
                  }
            Return Me.PostLoginMethod(loginQueryData, Me.cookieCollection1)
 
        Else
            Return False
 
        End If ' Me.GetMethod
 
    End Function
 

Traducción online a C# (sin testear):

/// <summary>
/// Gets the main url.
/// </summary>
/// <value>The main url.</value>
public string UrlMain {
	get { return "http://www.cgwallpapers.com/"; }
}
 
/// <summary>
/// Gets the login url.
/// </summary>
/// <value>The login url.</value>
public string UrlLogin {
	get { return "http://www.cgwallpapers.com/login.php"; }
}
 
/// <summary>
/// Gets the login query string format.
/// </summary>
/// <value>The login query string format.</value>
public string UrlLoginQueryFormat {
	get { return "usuario={0}&contrasena={1}"; }
}
 
/// <summary>
/// Gets the headers for a Login POST request.
/// </summary>
/// <value>The headers for a Login POST request.</value>
public WebHeaderCollection RequestHeadersPostLogin {
 
	get {
		WebHeaderCollection headers = new WebHeaderCollection();
		var _with1 = headers;
		_with1.Add("Accept-Language", "en-us,en;q=0.5");
		_with1.Add("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");
		_with1.Add("Keep-Alive", "99999");
		return headers;
 
	}
}
 
/// <summary>
/// Determines whether the user is logged in the site.
/// </summary>
 
private bool isLogged;
/// <summary>
/// Gets the cookie container.
/// </summary>
/// <value>The cookie container.</value>
public CookieCollection CookieCollection {
	get { return this.cookieCollection1; }
}
/// <summary>
/// The cookie container.
/// </summary>
 
private CookieCollection cookieCollection1;
/// <summary>
/// Defines the query data for a LoginPost request.
/// </summary>
private sealed class LoginQueryData
{
 
	/// <summary>
	/// Gets the Usuario field.
	/// </summary>
	/// <value>The Usuario field.</value>
	public string Usuario { get; set; }
 
	/// <summary>
	/// Gets or sets the Conteasena field.
	/// </summary>
	/// <value>The Conteasena field.</value>
	public string Contrasena { get; set; }
 
}
 
/// <summary>
/// Gets a formatted <see cref="String"/> representation of a <see cref="LoginQueryData"/> object.
/// </summary>
/// <param name="loginQueryData">The <see cref="LoginQueryData"/> object that contains the login query fields.</param>
/// <returns>A formatted <see cref="String"/> representation of a <see cref="LoginQueryData"/> object.</returns>
private string GetLoginQueryString(LoginQueryData loginQueryData)
{
 
	return string.Format(this.UrlLoginQueryFormat, loginQueryData.Usuario, loginQueryData.Contrasena);
 
}
 
/// <summary>
/// Sets the cookie container.
/// </summary>
/// <param name="url">The url.</param>
/// <param name="cookieCollection">The cookie collection.</param>
/// <returns>CookieContainer.</returns>
private CookieContainer SetCookieContainer(string url, CookieCollection cookieCollection)
{
 
	CookieContainer cookieContainer = new CookieContainer();
	System.DateTime refDate = default(System.DateTime);
 
 
	foreach (Cookie oldCookie in cookieCollection) {
 
		if (!DateTime.TryParse(oldCookie.Value, refDate)) {
			Cookie newCookie = new Cookie();
			var _with2 = newCookie;
			_with2.Name = oldCookie.Name;
			_with2.Value = oldCookie.Value;
			_with2.Domain = new Uri(url).Host;
			_with2.Secure = false;
 
			cookieContainer.Add(newCookie);
 
		}
 
	}
 
	return cookieContainer;
 
}
 
/// <summary>
/// Converts cookie string to global cookie collection object.
/// </summary>
/// <param name="cookie">The cookie string.</param>
/// <param name="cookieCollection">The cookie collection.</param>
 
private void SaveCookies(string cookie, ref CookieCollection cookieCollection)
{
	string[] cookieStrings = cookie.Trim.Replace("path=/,", string.Empty).Replace("path=/", string.Empty).Split({ ';' }, StringSplitOptions.RemoveEmptyEntries);
 
	cookieCollection = new CookieCollection();
 
 
	foreach (string cookieString in cookieStrings) {
 
		if (!string.IsNullOrEmpty(cookieString.Trim)) {
			cookieCollection.Add(new Cookie(name: cookieString.Trim.Split('=')(0), value: cookieString.Trim.Split('=')(1)));
 
		}
 
	}
 
}
 
/// <summary>
/// Convert cookie container object to global cookie collection object.
/// </summary>
/// <param name="cookieContainer">The cookie container.</param>
/// <param name="cookieCollection">The cookie collection.</param>
/// <param name="url">The url.</param>
 
private void SaveCookies(CookieContainer cookieContainer, ref CookieCollection cookieCollection, string url)
{
	cookieCollection = new CookieCollection();
 
 
	foreach (Cookie cookie in cookieContainer.GetCookies(new Uri(url))) {
		cookieCollection.Add(cookie);
 
	}
 
}
 
/// <param name="url">The url.</param>
/// <param name="cookieCollection">The cookie collection.</param>
/// <returns><c>true</c> if successfull, <c>false</c> otherwise.</returns>
private bool GetMethod(string url, ref CookieCollection cookieCollection)
{
 
	Debug.WriteLine("[+] GetMethod function started.");
 
	HttpWebRequest request = null;
	HttpWebResponse response = null;
	StreamReader sr = null;
	bool result = false;
 
	try {
		Debug.WriteLine("[+] Attempting to perform a request with:");
		Debug.WriteLine(string.Format("Method: {0}", "GET"));
		Debug.WriteLine(string.Format("Headers: {0}", string.Join(Environment.NewLine, this.RequestHeadersPostLogin)));
 
		request = (HttpWebRequest)HttpWebRequest.Create(url);
		var _with3 = request;
		_with3.Method = "GET";
		_with3.Headers = this.RequestHeadersPostLogin;
		_with3.Accept = "text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5";
		_with3.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0";
		_with3.AllowAutoRedirect = false;
		_with3.KeepAlive = true;
		Debug.WriteLine("[-] Request done.");
 
		// Get the server response.
		Debug.WriteLine("[+] Getting server response...");
		response = (HttpWebResponse)request.GetResponse;
		Debug.WriteLine("[-] Getting server response done.");
 
 
		if (request.HaveResponse) {
			// Save the cookie info.
			Debug.WriteLine("[+] Saving cookies...");
			this.SaveCookies(response.Headers("Set-Cookie"), ref cookieCollection);
			Debug.WriteLine("[-] Saving cookies done.");
 
			// Get the server response.
			Debug.WriteLine("[+] Getting server response...");
			response = (HttpWebResponse)request.GetResponse;
			Debug.WriteLine("[-] Getting server response done.");
 
			Debug.WriteLine("[+] Reading server response...");
			sr = new StreamReader(response.GetResponseStream);
			using (sr) {
				// Read the response from the server, but we do not save it.
				sr.ReadToEnd();
			}
			result = true;
			Debug.WriteLine("[-] Reading server response done.");
 
		// No response received from server.
		} else {
			throw new Exception(string.Format("No response received from server with url: {0}", url));
			result = false;
 
		}
 
	} catch (Exception ex) {
		throw;
		result = false;
 
	} finally {
		if (sr != null) {
			sr.Dispose();
		}
		if (response != null) {
			response.Close();
		}
 
	}
 
	Debug.WriteLine("[-] GetMethod function finished.");
	Debug.WriteLine("[i] Returning result value...");
	return result;
 
}
 
/// <param name="loginData">The login post data.</param>
/// <param name="cookieCollection">The cookie collection.</param>
/// <returns><c>true</c> if successfull, <c>false</c> otherwise.</returns>
private bool PostLoginMethod(LoginQueryData loginData, ref CookieCollection cookieCollection)
{
 
	Debug.WriteLine("[+] PostLoginMethod function started.");
 
	HttpWebRequest request = null;
	HttpWebResponse response = null;
	StreamWriter sw = null;
	int initialCookieCount = 0;
	string postData = null;
	bool result = false;
 
	try {
		Debug.WriteLine("[+] Attempting to perform a login request with:");
		Debug.WriteLine(string.Format("Method: {0}", "POST"));
		Debug.WriteLine(string.Format("Referer: {0}", this.UrlMain));
		Debug.WriteLine(string.Format("Accept: {0}", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"));
		Debug.WriteLine(string.Format("ContentType: {0}", "application/x-www-form-urlencoded"));
		Debug.WriteLine(string.Format("Headers: {0}", string.Join(Environment.NewLine, this.RequestHeadersPostLogin)));
 
		request = (HttpWebRequest)HttpWebRequest.Create(this.UrlLogin);
		var _with4 = request;
		_with4.Method = "POST";
		_with4.Headers = this.RequestHeadersPostLogin;
		_with4.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
		_with4.ContentType = "application/x-www-form-urlencoded";
		_with4.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0";
		_with4.Referer = this.UrlMain;
		Debug.WriteLine("[-] Request done.");
 
		Debug.WriteLine("[+] Passing request cookie info...");
		// Pass cookie info from the login page.
		if (cookieCollection != null) {
			request.CookieContainer = this.SetCookieContainer(this.UrlLogin, cookieCollection);
		}
		Debug.WriteLine("[-] Passing request cookie info done.");
 
		// Set the post data.
		Debug.WriteLine("[+] Setting post data with:");
		Debug.WriteLine(string.Format("Usuario: {0}", loginData.Usuario));
		Debug.WriteLine(string.Format("Contrasena: {0}", loginData.Contrasena));
		postData = this.GetLoginQueryString(loginData);
 
		sw = new StreamWriter(request.GetRequestStream);
		using (sw) {
			sw.Write(postData);
			// Post the data to the server.
		}
		Debug.WriteLine("[-] Setting post data done.");
 
		// Get the server response.
		Debug.WriteLine("[+] Getting server response...");
		initialCookieCount = request.CookieContainer.Count;
		response = (HttpWebResponse)request.GetResponse;
		Debug.WriteLine("[-] Getting server response done.");
 
		// Login successful.
		if (request.CookieContainer.Count > initialCookieCount) {
			result = true;
 
		// Login unsuccessful.
		} else {
			result = false;
 
		}
 
		Debug.WriteLine(string.Format("[i] Login response result is: {0}", result ? "Successful (True)" : "Unsuccessful (False)"));
 
		// Save new login cookies.
		if (result) {
			Debug.WriteLine("[+] Saving new login cookies...");
			this.SaveCookies(request.CookieContainer, ref cookieCollection, this.UrlMain);
			Debug.WriteLine("[-] Saving new login cookies done.");
		}
 
	} catch (Exception ex) {
		throw;
 
	} finally {
		if (sw != null) {
			sw.Dispose();
		}
		if (response != null) {
			response.Close();
		}
 
	}
 
	Debug.WriteLine("[-] PostLoginMethod function finished.");
	Debug.WriteLine("[i] Returning result value...");
	this.isLogged = result;
	return result;
 
}
 
 
/// <summary>
/// Determines whether the account can log in CGWallpapers site.
/// </summary>
/// <returns><c>true</c> if the account can log in CGWallpapers site, <c>false</c> otherwise.</returns>
public bool CheckLogin(string username, string password)
{
 
 
	if (this.GetMethod(this.UrlMain, this.cookieCollection1)) {
		LoginQueryData loginQueryData = new LoginQueryData {
			Usuario = HttpUtility.UrlEncode(username),
			Contrasena = HttpUtility.UrlEncode(password)
		};
		return this.PostLoginMethod(loginQueryData, this.cookieCollection1);
 
	} else {
		return false;
 
	}
	// Me.GetMethod
 
}
 
//=======================================================
//Service provided by Telerik (www.telerik.com)
//=======================================================

Por codificar me refiero a la codificación html, no a codificar el string en una secuencia de bytes cómo estás haciendo.

Si el usuario/contraseña contiene por ejemplo un espacio entonces vas a codificar 1 byte " " en lugar de codificar los caracteres "%20", ¿me entiendes?.

Porfavor, aclara si el usuario/contraseña contienen espacios u otros caracteres ilegales para descartar que ese sea el problema,
si tienes dudas vuelve a leer la pregunta nº1 que hice en el comentario anterior donde mostré un ejemplo de cómo codificar el string (primero codificas el usuario/contraseña a html, y luego generas los bytes cómo estabas haciendo, pero con el usuario/contraseña codificados a html).

Saludos!

Muy buenas he visto que estabas en apuros y esto ya lo discutimos hace un tiempo. Ese code en C# me suena jejej


¿Quieres decir que cuando introduces el user/passwd correcto te lleva al index? Yo lo que suelo hacer para construir peticiones HTTP es analizarlas primero con un FireBug en Firefox para determinar las cabeceras de la solicitud y POST (en este caso, pero lo mismo con GET). Dale una oportunidad y nos cuentas. Cuanta mas info nos des mejor.

Fíjate que el header "Host" no lo tienes seteado a la web que lanzas el POST. Quizá lo quisiste poner así en este post, pero lo remarco por si acaso.

req.Host = "www.web.com"

Saludos.