Sounds like you need a Codec Pac. You could try a different Media Player. VLC Media Player has Codecs Built in and is Highly recommended here on the Forums. 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 put together for several solutions that may help.
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers