Sounds like a CODEC issue or a graphics card problem. Get GSpot.. it's free and it will tell you what you need to play a certain file.. just open the file in GSpot and it will tell you. http://www.headbands.com/gspot/

Free-Codecs.com

Consider a backup player with built-in codecs such as VLC; http://www.videolan.org/vlc/ (choose nearest mirror site)