public
  { Public declarations }
  procedure GrabScreen;
...

implementation
{$R *.DFM}

procedure TForm1.GrabScreen;
var
  DeskTopDC: HDc;
  DeskTopCanvas: TCanvas;
  DeskTopRect: TRect;
begin
  DeskTopDC := GetWindowDC(GetDeskTopWindow);
  DeskTopCanvas := TCanvas.Create;
  DeskTopCanvas.Handle := DeskTopDC;
  DeskTopRect := Rect(0, 0, Screen.Width, Screen.Height);
  Form1.Canvas.CopyRect(DeskTopRect, DeskTopCanvas, DeskTopRect);
  ReleaseDC(GetDeskTopWindow, DeskTopDC);
end;


procedure TForm1.Button1Click(Sender: TObject);
begin
  GrabScreen;
end;