First, make sure you have the latest nVidia drivers for that MX 420, if you don't, install the current version and see if that helps. If that doesn't help, since I don't have any more info, the problem could be due to the fact that the Mac you're using has a better video card than the one on your PC, as the MX 420 is a rather low end card.\n\nYou can get the latest drivers here:  http://www.nvidia.com/page/home.html