You might need codecs for WMP.

Check out this thread (click on link): CODECs list for Movies/Audio/Alternate players / Video help forums/ video-mp3 fixers

Lots of people use the K-Lite codecs. Otherwise, there are heaps more to choose from in that thread. I've used the XP codec pack in the past.

Just remember to only have one codec pack installed at a time, otherwise you can run into problems.

Or you could try VLC player, which is a free player & has codecs installed to play most file types. See the thread at the top of this post for a link to download VLC