Maybe you need codecs for the player Have a look here for links to different codec packs. The K-Lite & XP codec packs are popular.
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers

If you're using Vista, try this one.
Vista Codec Package v4.4.7 Beta 4a (freeware)

Or you could try using VLC player. It's free, has codecs installed & it's very reliable...plays most things
VLC media player - Overview