Sounds like a codecs issue. If you're using windows get either of these: http://www.land.ee/d_nimo_codec.html or http://www.free-codecs.com/

And get yourself a backup player like VLC which is free & has built-in codecs: http://www.videolan.org/vlc/ (choose nearest mirror site) DivX player might also be handy.

I suppose also see this: Setting Default video app

If you're using Mac please tell us.