Nejak takto ?:

var DC: HDC;
    Canvas: TCanvas;
begin
  Canvas:=TCanvas.Create;
  try
    DC:=GetWindowDC(GetDesktopWindow);
    try
      Canvas.Handle:=DC;
      //
      // teraz si skúmaj čo chceš...



    finally
      Canvas.Handle:=0;
      ReleaseDC(0,DC):
    end;
  finally
    Canvas.Free;
  end;
end;