{We'll need several global variables}

var

    Form1: TForm1;

    StartButton : hWnd;

    OldBitmap : THandle;

    NewImage : TPicture;

...

{Now, we put this code in the

OnCreate event of the form:}

procedure TForm1.FormCreate(Sender: TObject) ;

begin

  NewImage:=TPicture.create;

  NewImage.LoadFromFile('C:WindowsCircles.BMP') ;

  StartButton := FindWindowEx

                  (FindWindow(

                     'Shell_TrayWnd', nil),

                      0,'Button', nil) ;

   OldBitmap:=SendMessage(StartButton,

                          BM_SetImage, 0,

                          NewImage.Bitmap.Handle) ;

  end;

...

{And this other in the OnDestroy:}

  procedure TForm1.FormDestroy(Sender: TObject) ;

  begin

   SendMessage(StartButton,BM_SetImage,0,OldBitmap) ;

   NewImage.Free;

  end;