LW's Media player is Horrible at Best.
I Feel the same way about WMP, but there is a Codec Pack for Vista in the Link Below.
You could try a different Media Player. VLC Media Player has Codecs Built in and is Highly recommended here on the Forums. VLC is primarily a Video Player, but will Play Most Audio Files as well.
If you want to keep the Media Player that you are using then download only ONE Codec Pac as more than ONE may cause Problems.
See the Link Below that LOTR's put together for other options that may help.
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers
Hope this Helps.

__________________
I Don't Suffer From Insanity, I Enjoy Every Minute of it