Tja. Inget säger väl att fönster i windows måste vara fyrkantiga. Prova gärna följande:
1. Skapa ett vanligt EXE program i VB.
2. Set formens ScaleMode property till 3 - Pixel
3. Sätt in en Combobox mitt på fönstret
4. Lägg till en Module
5. Sätt in följande kod i modulen
<div class=code>
' API POINT med x och y 
Type POINTAPI 
 X As Long 
 Y As Long 
End Type 
 
' Konstanter som används för kombinationer 
Public Const RGN_AND = 1 
Public Const RGN_OR = 2 
Public Const RGN_XOR = 3 
Public Const RGN_DIFF = 4 
Public Const RGN_COPY = 5 
 
Declare Function CombineRgn Lib "gdi32" (ByVal hDestRgn As Long, _ 
 ByVal hSrcRgn1 As Long, _ 
 ByVal hSrcRgn2 As Long, _ 
 ByVal nCombineMode As Long) As Long 
 
Declare Function CreateRectRgn Lib "gdi32" (ByVal X1 As Long, _ 
 ByVal Y1 As Long, _ 
 ByVal X2 As Long, _ 
 ByVal Y2 As Long) As Long 
 
Declare Function CreateEllipticRgn Lib "gdi32" (ByVal X1 As Long, _ 
 ByVal Y1 As Long, _ 
 ByVal X2 As Long, _ 
 ByVal Y2 As Long) As Long 
 
Declare Function SetWindowRgn Lib "User32" (ByVal hWnd As Long, _ 
 ByVal hRgn As Long, _ 
 ByVal bRedraw As Boolean) As Long 
</div>

6. Sätt in följande kod i formen
<div class=code>
Private Sub Combo1_Click() 
 Dim resultat As Integer 
 Dim kombination As Long 
 Dim storRund As Long 
 Dim titelFyrkant As Long 
 Dim fyrkantCirkel As Long 
 Dim combonsBox As Long 
 Dim alltKombin As Long 
 Select Case Combo1.Text 
 Case "RGN_AND" 
 kombination = RGN_AND 
 Case "RGN_COPY" 
 kombination = RGN_COPY 
 Case "RGN_DIFF" 
 kombination = RGN_DIFF 
 Case "RGN_OR" 
 kombination = RGN_OR 
 Case "RGN_XOR" 
 kombination = RGN_XOR 
 End Select 
 storRund = CreateEllipticRgn(0, 2, Me.ScaleWidth, Me.ScaleHeight - 2) 
 titelFyrkant = CreateRectRgn(2, 3, Me.ScaleWidth + 2, 21) 
 alltKombin = CreateRectRgn(1, 1, 1, 1) 
 combonsBox = CreateRectRgn(Combo1.Left + 2, _ 
 Combo1.Top + 22, _ 
 Combo1.Left + Combo1.Width + 2, _ 
 Combo1.Top + Combo1.Height + 22) 
 ' storleken inte viktig 
 fyrkantCirkel = CreateRectRgn(0, 0, 10, 30) 
 resultat = CombineRgn(fyrkantCirkel, titelFyrkant, storRund, kombination) 
 resultat = CombineRgn(alltKombin, fyrkantCirkel, combonsBox, RGN_OR) 
 resultat = SetWindowRgn(Me.hWnd, alltKombin, True) 
End Sub 
 
Private Sub Form_Load() 
 Combo1.AddItem "RGN_AND" 
 Combo1.AddItem "RGN_COPY" 
 Combo1.AddItem "RGN_DIFF" 
 Combo1.AddItem "RGN_OR" 
 Combo1.AddItem "RGN_XOR" 
End Sub 
</div>

6. Prova genom att välja i Combon