type

  TForm1 = class(TForm)

    procedure FormCreate(Sender: TObject);

  private

    { Private declarations }

    procedure AppMessage( var Msg: TMsg; var handled: Boolean );

  end;

procedure TForm1.AppMessage( var Msg: TMsg; var handled: Boolean );

begin

   if ( Msg.Message = WM_SYSCOMMAND ) and ( Msg.wParam = SC_SCREENSAVE ) then

      Handled := True;

end;

procedure TForm1.FormCreate(Sender: TObject);

begin

   Application.OnMessage := AppMessage;

end;