CDex will convert WMA to MP3. I believe it supports drag and drop. Therefore, just search your music folder for *.wma, drag all the results into the CDex pane, select them all and convert. I don't know where it will deposit them, though. You might have to move them from a newly created folder back to where you want them, but it might copy them into their existing location. Regardless, it sounds like that's going to be your best option. And it is free.