I had a hard time finding out how to convert them into MP3, but once I'd figured it out I somehow found this article: convert wmv to mp3. I swear the Google search didn't work 3 weeks ago!
In any case, this is how I did it using stuff built into XP and iTunes for Windows:
1. Open in XP's built in Windows Movie Maker. Yes, it's on your drive! Right click on a WMV file and choose Edit.Update 1/17/08: I think MovieMaker may have changed since I first wrote this post. The trick now is epxlained here. Drag the imported movie to the Audio/Music timeline (NOT the audio timeline -- the post is wrong there). Now choose Save Movie File, then "My Computer" then "Show More Choices" link then "Other Settings" then choose "High Quality Audio".
2. Add as a clip to Movie Maker. Despite step #1 you need to do this manually.
3. Save move, but choose the audio-format only (WMA). Choose as high as you can because you're going to encode it again. (Click link to see options)
4. In iTunes, using advanced setup options, change iTunes import from AAC to 64kbps VBR MP3. (see also)
5. Drag and drop to iTunes. iTunes will conver to MP3. Drag and drop from desktop to upload as desired.