public
 { Public declarations }
 procedure GrabScreen;
...
implementation
{$R *.DFM}
procedure TForm1.GrabScreen;
var
 DeskDC: HDc;
 DeskCanvas: TCanvas;
 DeskRect: TRect;
begin
 DeskDC := GetWindowDC(GetDeskWindow);
 DeskCanvas := TCanvas.Create;
 DeskCanvas.Handle := DeskDC;
 DeskRect := Rect(0, 0, Screen.Width, Screen.Height);
 Form1.Canvas.CopyRect(DeskRect, DeskCanvas, DeskRect);
 ReleaseDC(GetDeskWindow, DeskDC);
end;

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