Making a transparent form

Add a Button (Button1) to a Delphi form (Form1)
procedure TForm1.FormCreate(Sender: TObject) ;
var
FullRgn, ClientRgn, ButtonRgn: THandle;
Margin, X, Y: Integer;
begin
Margin := (Width - ClientWidth) div 2;
FullRgn := CreateRectRgn(0, 0, Width, Height) ;
X := Margin;
Y := Height - ClientHeight - Margin;
ClientRgn := CreateRectRgn
(X, Y, X + ClientWidth, Y + ClientHeight) ;
CombineRgn(FullRgn, FullRgn, ClientRgn, RGN_DIFF) ;
X := X + Button1.Left;
Y := Y + Button1.Top;
ButtonRgn := CreateRectRgn
(X, Y, X + Button1.Width, Y + Button1.Height) ;
CombineRgn(FullRgn, FullRgn, ButtonRgn, RGN_OR) ;
SetWindowRgn(Handle, FullRgn, True) ;
end;

from: about.com