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;