Media Player 10 now has the option of ripping in MP3.
May be worth downloading just for that feature. I
haven't figured out if there's a way to convert all your
existing files (like in iTunes and others).
Microsoft wants to charge you $9.95 for a plug-in which enables ripping to
mp3 format. DON'T DO IT!!! Instead, use the excellent *freeware* ripper
CDex: http://cdexos.sourceforge.net/
