or if you dont want to do that (and by the way, doing that will not remove any of the tags associated with itunes music store, i think it only removes the DRM protection)
if you have a version earlier than itunes 6 you can use a program called Hymm. i dont feel like looking for the website, but it romves all DRM protection and any tags associated with the Itunes music store. if you dont remove ALL tags, and itunes/apple/RIAA really feel like being assholes, they can still find out that the song you are playing was a song you bought from itunes.