procedure TForm1.Button1Click(Sender: TObject); 
var
  DeskTopDC: HDc; 
 DeskTopCanvas: TCanvas; 
 DeskTopRect: TRect; 
 Bitmap: TBitmap; 
begin 
 DeskTopDC := GetWindowDC(GetDeskTopWindow);
 DeskTopCanvas := TCanvas.Create; 
 DeskTopCanvas.Handle := DeskTopDC; 
 DeskTopRect := Rect(0,0,Screen.Width,Screen.Height); 
 Bitmap := TBitmap.Create; 
 with Bitmap do 
 begin 
 Width := Screen.Width; 
 Height:= Screen.Height; 
 PixelFormat := pfDevice; 
 end; 
 Bitmap.Canvas.CopyRect(DeskTopRect,DeskTopCanvas,DeskTopRect); 
 Bitmap.SaveToFile ('sample.bmp'); 
 Bitmap.Free; 
 DesktopCanvas.Free; 
 ReleaseDC(GetDeskTopWindow,DeskTopDC); 
end;