Are you using the latest versions of DirectX and Windows Media Player? You may just want to stroll on over to http://www.windowsupdate.com
and make sure it's all up to date.
Also, do you have any 3rd party codec packs installed? Like, those crazy all-in-one codecs that include a ton of conflicting codecs? If so, uninstall all of them. I've personally removed everything (except XviD and DivX, but only for encoding purposes) and I use a program called ffdshow audio. It's like, a VfW decoder or something. I forget the exact term. Point is I put it on almost every PC I come across so people don't have to worry about crap with various codecs.