If Resultado = Windows.Forms.DialogResult.OK Then
               ' Aquí iria el codigo
        End If
 

Option Explicit On
 
Public Class Form2
 
    Dim Listita As New ListBox
    Dim FBD As New FolderBrowserDialog
 
 
    Private Sub Form2_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
        Listita.BackColor = Color.White
        Listita.ForeColor = Color.Black
        Listita.Size = New Point(340, 251)
        Listita.Location = New Point(12, 25) ' Dejo un margen de espacio de 100 px porque solo quieres ocupar "la mayor parte"
        AddHandler Listita.SelectedIndexChanged, AddressOf Listita_SelectedIndexChanged
        Me.Controls.Add(Listita)
 
        Dim Folder As String = "C:\Users\Alvaro\Documents"
        Dim Files As Array = IO.Directory.GetFiles(Folder, "*", IO.SearchOption.TopDirectoryOnly)
 
        For Each File In Files
            Dim File_Info As New IO.FileInfo(File)
            Listita.Items.Add(File_Info.Name)
        Next
 
        Dim Files1 As Array = IO.Directory.GetFiles(Environment.GetFolderPath(Environment.SpecialFolder.System), "*", IO.SearchOption.TopDirectoryOnly)
        Dim TotalRoms As Int32 = Files1.Length
        Label1.Text = "Roms encontrados (" & TotalRoms & ")"
    End Sub
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Me.Close()
    End Sub
 
    Private Sub Listita_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim Item As String = Listita.SelectedItem
        MsgBox(Folder + Item)
    End Sub
 
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        FBD.Description = " Seleccionar una carpeta "
        FBD.SelectedPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)
        FBD.ShowNewFolderButton = True
        Dim Folder As String = FBD.SelectedPath
        Dim Resultado As DialogResult = FBD.ShowDialog
        If Resultado = Windows.Forms.DialogResult.OK Then
 
        End If
 
        FBD.Dispose()
    End Sub
End Class