procedure ScreenToFile(aFileName: string);
var
  Bitmap: TBitmap;
  Canvas: TCanvas;
begin
  Canvas := TCanvas.Create;
  try
    Canvas.Handle := GetWindowDC(0);
    try
      Bitmap := TBitmap.Create;
      try
        Bitmap.Width := Screen.DesktopWidth;
        Bitmap.Height:= Screen.DesktopHeight;
        Bitmap.PixelFormat := pfDevice;
        BitBlt(Bitmap.Canvas.Handle, 0, 0, Pred(Screen.DesktopWidth), Pred(
            Screen.DesktopHeight), Canvas.Handle, Screen.DesktopLeft,
            Screen.DesktopTop, Bitmap.Canvas.CopyMode);
        Bitmap.SaveToFile(aFileName);
      finally
        Bitmap.Free;
      end;
    finally
      ReleaseDC(0, Canvas.Handle);
    end;
  finally
    Canvas.Free;
  end;
end;