If it's running an OS newer than Windows 7, chances are the MPEG-2 decoder is not installed on your system. Essentially, MPEG-2 is the video codec used by DVDs, but it's saddled with patents and other legal things I don't understand that make it expensive to implement. Microsoft decided to stop providing this codec to save money, so I'd suggest using a different player like MPC-HC or VLC, as these players have their own decoders that are freely available (again, due to legal stuff I don't understand).