http://www.dbpoweramp.com/dmc.htm
best thing ive found for converting music on the fly, has a nice popup feature in Explorer shows all the compression info, ratio, %, bitrate, codec used... that kinda thing.
takes some patience to install all the codecs, but once its on, will do any file type you want :-D even monkey!
it removes most codecs from initial install (has mp3 as standard) because of all the proprietry codecs around.