allora prova così (mi spiace non posso testarlo)

codice:
Private Declare Function GetWindowLong Lib "User32" Alias "GetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long) As Long

Private Declare Function SetWindowLong Lib "User32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long

Private Declare Function SetLayeredWindowAttributes Lib "User32" (ByVal hwnd As Long, ByVal crey As Byte, ByVal bAlpha As Byte, ByVal dwFlags As Long) As Long

Private Const GWL_EXSTYLE = (-20)
Private Const WS_EX_LAYERED = &H80000
Private Const LWA_ALPHA = &H2&


Private Sub Command1_Click()
Dim LEVEL As Byte
LEVEL = 120
Call SetWindowLong(Me.hwnd, GWL_EXSTYLE, GetWindowLong(Me.hwnd, GWL_EXSTYLE) Or WS_EX_LAYERED)

Call SetLayeredWindowAttributes(Me.hwnd, 0, LEVEL, LWA_ALPHA)

End Sub
per documentazione
http://msdn.microsoft.com/library/de...Attributes.asp