Open whatever folder you store your files in. The default location is C:\documents and Settings\"your account name"\Shared. Right click on a file>click Properties>Opens with "change"> choose "windows media player" click ok. That should change your files back. If any files didn't change, right click on that file and repeat.
There's more info in this thread:
http://www.gnutellaforums.com/showth...threadid=34465