I've tried without success - I'm obviously missing something. I'm in the "Library screen" with the tracks listed and then followed your instructions; I've selected "mp3 format" under the Rip Music tab clicked OK, but nothing happens. I can't find an option to actually "rip" the tracks / files themselves. All the files in the "My Music" folder remain in wma format.
Sorry for appearing daft, but are you able to give me a step guide to change the format, so that I end up with mp3 files?
I've downloaded ITunes, but still no joy! None of the wma files appear in the I Tunes Music Library (although the existing mp3 ones do) Is there a way I can get the wma ones into the library to convert? They presumably need to show in the Library before I can follow the Edit - Preferences route?