My first suggestion would be to get rid of WMP. The threads below will give you Codec options and also an alternitive Media Player which is very highly thought of here on the forums. See Links below.
Have a look at the thread below for links to heaps of codec packs (some people like the XP codec pack for WMP & the K-lite pack's popular too).
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers
That thread also has a link to VLC player. It's free, has codecs installed & is very reliable.
And here's a link for Vista codecs, just in case you're using Vista.
Vista Codec Package v4.4.8 (freeware).
Hope this helps.
