function SetFullscreenMode: Boolean;
var
 DeviceMode: TDevMode;
begin
 with DeviceMode do
 begin
  dmSize := SizeOf(DeviceMode);
  dmBitsPerPel := 16;
  dmPelsWidth := 640;
  dmPelsHeight := 480;
  dmFields := DM_BITSPERPEL or DM_PELSWIDTH or DM_PELSHEIGHT;
  result := False;
  if ChangeDisplaySettings(DeviceMode, CDS_TEST or CDS_FULLSCREEN) <>
  DISP_CHANGE_SUCCESSFUL then
  Exit;
  Result := ChangeDisplaySettings(DeviceMode, CDS_FULLSCREEN) =
  DISP_CHANGE_SUCCESSFUL;
 end;
end;
procedure RestoreDefaultMode;
var
 T: TDevMode absolute 0;
begin
 ChangeDisplaySettings(T, CDS_FULLSCREEN);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
 if setFullScreenMode then
 begin
  sleep(7000);
  RestoreDefaultMode;
 end;
end;