Sounds like you need xtra codecs. Without knowing which operating system you use it makes it a bit difficult. I'll guess you're using windows so try here: http://www.divx-digest.com/software/nimo_pack.html

Or else you can try another player which has built in codecs & plays all video files except the odd wmv file. VLC (choose the closest mirror site to where you are): http://www.videolan.org/vlc/
(there's versions for both windows & mac.)