In reply to hamsforlegs:
Yes, MediaMonkey will do it, even the free version*. Import your music, sort by file type, select all the WMA files, and use Tools/Convert format to MP3. Set a suitable compression level; personally, I'd go for the maximum available, regardless of the original source quality. WMA is generally considered to be a more efficient format, so that a 128kbps WMA is roughly sonically equivalent to a 192kbps MP3.
Be aware that transcoding inevitably reduces audio quality, since you're introducing a second set of compression artefacts.
* MM comes with a '30 day trial of LAME'. Which is kind of a cheek, since LAME is free, and not developed by Ventis Media. But you can get aroung the limit when it expires, by deleting the lame_enc.dll file from the install directory, starting MM, trying to use LAME, closing MM and then downloading and installing lame_enc.dll from the LAME development site.