It's possible it's a codec issue. You may need to update your codecs. Try another player with built-in codecs to see if the same thing happens. If it does then the file might have problems. Try VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)

Codecs: http://www.land.ee/d_nimo_codec.html OR http://www.click-now.net/html/K-LiteCodecPack.htm

Another player to try is XDiv player. Google it.