Which codecs do you have installed?
I suggest you install the Shark007's Codec Pack: Shark007.net - Windows 7 Codecs - WMP12 Codecs
Those are some really good codecs, and a lot of users here on the forums agree with this
Those Codecs will give you the ability to play almost everything you throw at the WMP.
Remember to uninstall every other codec pack you may have in your PC before installing Shark007's Codec Pack.