Try VLC player...it's free, has codecs installed & can play most file types
VLC media player - Overview


If you want to use WMP & have XP, try installing this codec pack.
Download XP Codec Pack 2.0.8 - XP Codec Pack is one of the most complete codec pack

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