Delphi

{ define Global vars }

var
Form1: TForm1;
StartButton: hWnd;
OldBitmap: THandle;
NewImage: TPicture;

{ put the Code in the OnCreate event of your form }

procedure TForm1.FormCreate(Sender: TObject);
begin
NewImage := TPicture.create;
NewImage.LoadFromFile('C:\Windows\Circles.BMP');
StartButton := FindWindowEx
(FindWindow(
'Shell_TrayWnd', nil),
0,'Button', nil);
OldBitmap := SendMessage(StartButton,
BM_SetImage, 0,
NewImage.Bitmap.Handle);
end;

{ OnDestroy-Event }

procedure TForm1.FormDestroy(Sender: TObject);
begin
SendMessage(StartButton,BM_SetImage,0,OldBitmap);
NewImage.Free;
end;