/修改

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

/ 将代码放入FormCreate事件

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;

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

for win98se/Delphi 5.0通过。