{Designed by http://delphi.ktop.com.tw}
procedure TForm1.BitBtn1Click(Sender: TObject);
var
  DC: HDC;
  Canvas: TCanvas;
  MyBitmap: TBitmap;
begin
  Canvas := TCanvas.Create;
  MyBitmap := TBitmap.Create;
  DC := GetDC(0);
  try
    Canvas.Handle := DC;
    with Screen do
    begin
      MyBitmap.Width := Width;
      MyBitmap.Height := Height;
      MyBitmap.Canvas.CopyRect(Rect(0, 0, Width, Height), Canvas,
                               Rect(0, 0, Width, Height));
      MyBitmap.SaveToFile('c:\test.bmp');
    end;
  finally
    ReleaseDC(0, DC);
    MyBitmap.Free;
    Canvas.Free
  end;
end;