[NumLock]

Код:

procedure ChangeDisplayResolution(x,y:word);
var dm : TDEVMODE;
begin
  ZeroMemory(@dm, sizeof(TDEVMODE));
  dm.dmSize := sizeof(TDEVMODE);
  dm.dmPelsWidth := x;
  dm.dmPelsHeight := y;
  dm.dmFields := DM_PELSWIDTH or DM_PELSHEIGHT;
  ChangeDisplaySettings(dm, 0);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ChangeDisplayResolution(800, 600);
end;