YOu could also try this: VLC: http://www.videolan.org/vlc/ (choose the nearest mirror site) It has inbuilt codecs & plays everything except the odd wmv file. It also plays some audio formats. Not the prettiest of players or the most stable but it works & is effective.

Otherwise make sure you have needed codecs: http://www.divx-digest.com/software/nimo_pack.html