procedure CaptureScreen(WholeDesktop: Boolean);
var
  Desktop: HWND;
  DesktopDC: HDC;
  FWidth, FHeight, FLeft, FTop: Integer;
  AImage: TBitmap;
  WindowRect: TRect;
  AFormat: Word;
  AData, APalette : THandle;

begin
  Desktop := GetDesktopWindow;

  AImage := TBitmap.Create;

  If WholeDesktop then begin
    FWidth  := Screen.Width;
    FHeight := Screen.Height;
    FLeft   := 1;
    FTop    := 1;

  end else begin
    GetWindowRect (GetForegroundWindow, WindowRect);
    FWidth  := WindowRect.Right - WindowRect.Left;
    FHeight := WindowRect.Bottom - WindowRect.Top;
    FLeft   := WindowRect.Left;
    FTop    := WindowRect.Top;
  end;

  DesktopDC := GetWindowDC(Desktop);
  try
    AImage.Width := FWidth;
    AImage.Height := FHeight;
    BitBlt(AImage.Canvas.Handle, 1, 1, FWidth, FHeight,
      DesktopDC, FLeft, FTop, SRCCOPY);
    AImage.SaveToFile('c:\test.bmp');
    WriteToFileRect;
  finally
    ReleaseDC(Desktop, DesktopDC);
    AImage.Free;
  end;
end;

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