Public Const HKEY_CLASSES_ROOT = &H80000000
Public Const HKEY_CURRENT_USER = &H80000001
Public Const HKEY_LOCAL_MACHINE = &H80000002
Public Const HKEY_USERS = &H80000003
Public Const HKEY_CURRENT_CONFIG = &H80000005

Private Declare Function RegCloseKey Lib "advapi32.dll" (ByVal hKey As Long) As Long
Private Declare Function RegOpenKey Lib "advapi32.dll" Alias "RegOpenKeyA" (ByVal hKey As Long, ByVal lpSubKey As String, phkResult As Long) As Long


Public Function RegComprobarClave(Key As String, Subkey As String) As Boolean
    Dim regRes As Long
    RegOpenKey Key, Subkey, regRes
    If regRes = 0 Then
        RegComprobarClave = False
    Else
        RegComprobarClave = True
    End If
    RegCloseKey regRes
End Function

Key = LLave(Key)

MsgBox RegComprobarClave(HKEY_LOCAL_MACHINE, "Software\Microsoft")