If your downloads have mysteriously turned into limewire docs, Macamp can play your mp3s. Quicktime will play most movie files. If this isn't working, try renaming the files with the correct extension. The Mac is not my area of expertise, but I believe that you can change what file types are associated with what program. Here's a link to a thread where many have found successful fixes.

http://www.gnutellaforums.com/showth...&threadid=2733

Hope this helps! :-)