If you're using the LW player, disable it (tools > options > player. Untick 'use LW player' & apply). Make something else your default player.

If you're already using another player, it might be a problem with codecs. There's a link to different codecs here
http://www.gnutellaforums.com/showth...ght=wmp+codecs

http://www.gnutellaforums.com/showth...threadid=40665