' Give the control an elliptical region.
Private Sub UserControl_Resize()
Dim rgn As Long
Dim wid As Single
Dim hgt As Single

    ' Create the elliptical region.
    wid = ScaleX(Width, vbTwips, vbPixels)
    hgt = ScaleY(Height, vbTwips, vbPixels)
    rgn = CreateEllipticRgn(0, 0, wid, hgt)

    ' Restrict the window to the region.
    SetWindowRgn hWnd, rgn, True
    DeleteObject rgn
End Sub