Esta rutina permite que un trozo de nuestro formulario sea transparente y podamos acceder a los objetos que veamos en el "agujero".

Private Declare Function CreateRoundRectRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long, ByVal X3 As Long, ByVal Y3 As Long) As Long

Private Declare Function CreateRectRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long

Private Declare Function CreateEllipticRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long

Private Declare Function CombineRgn Lib "gdi32" (ByVal hDestRgn As Long, ByVal hSrcRgn1 As Long, ByVal hSrcRgn2 As Long, ByVal nCombineMode As Long) As Long

Private Declare Function SetWindowRgn Lib "user32" (ByVal hWnd As Long, ByVal hRgn As Long, ByVal bRedraw As Long) As Long

Private Function fMakeATranspArea(AreaType As String, pCordinate() As Long) As Boolean
    'Name: fMakeATranpArea
    'Author: Dalin Nie
    'Date: 5/18/98
    'Purpose: Create a Transprarent Area in
    ' a form so that you can see through
    'Input: Areatype : a String indicate wha
    ' t kind of hole shape it would like to ma
    ' ke
    ' PCordinate : the cordinate area needed


    ' for create the shape:
        ' Example: X1, Y1, X2, Y2 for Rectangle
        'OutPut: A boolean
        Const RGN_DIFF = 4
        Dim lOriginalForm As Long
        Dim ltheHole As Long
        Dim lNewForm As Long
        Dim lFwidth As Single
        Dim lFHeight As Single
        Dim lborder_width As Single
        Dim ltitle_height As Single
        On Error Goto Trap
        lFwidth = ScaleX(Width, vbTwips, vbPixels)
        lFHeight = ScaleY(Height, vbTwips, vbPixels)
        lOriginalForm = CreateRectRgn(0, 0, lFwidth, lFHeight)

        lborder_width = (lFHeight - ScaleWidth) / 2
        ltitle_height = lFHeight - lborder_width - ScaleHeight


        Select Case AreaType

            Case "Elliptic"

            ltheHole = CreateEllipticRgn(pCordinate(1), pCordinate(2), pCordinate(3), pCordinate(4))
            Case "RectAngle"

            ltheHole = CreateRectRgn(pCordinate(1), pCordinate(2), pCordinate(3), pCordinate(4))

            Case "RoundRect"

            ltheHole = CreateRoundRectRgn(pCordinate(1), pCordinate(2), pCordinate(3), pCordinate(4), pCordinate(5), pCordinate(6))
            Case "Circle"
            ltheHole = CreateRoundRectRgn(pCordinate(1), pCordinate(2), pCordinate(3), pCordinate(4), pCordinate(3), pCordinate(4))

            Case Else
            MsgBox "Unknown Shape!!"
            Exit Function
        End Select
    lNewForm = CreateRectRgn(0, 0, 0, 0)
    CombineRgn lNewForm, lOriginalForm, _
    ltheHole, RGN_DIFF

    SetWindowRgn hWnd, lNewForm, True
    Me.Refresh
    fMakeATranspArea = True
    Exit Function
    Trap:
    MsgBox "error Occurred. Error # " & Err.Number & ", " & Err.Description
End Function


Para llamar a la función :

Dim lParam(1 To 6) As Long
lParam(1) = 100
lParam(2) = 100
lParam(3) = 250
lParam(4) = 250
lParam(5) = 50
lParam(6) = 50
Call fMakeATranspArea("RoundRect", lParam())
'Call fMakeATranspArea("RectAngle", lParam())
'Call fMakeATranspArea("Circle", lParam())
'Call fMakeATranspArea("Elliptic", lParam())

Según está la función, cuando creamos una región transparente existiendo ya una esta se elimina pero no sería difícil modificar la función para que la conserve.