That's for initial ripping from CD. Open your music library in WMP, right click on a track or select several, select "send to" then Windows Audio converter. This will bring up a box with conversion options.
See click here and click on Free mp3 WMA Converter.
The free version will convert several hundred files at a time as long as they are all in the same folder. If you want more sophisticated options (e.g. converting a series of sub-folders), you can go for the paid-for version.
A much better solution to this is to use the free batch conversion too WinFF, which also converts tons of other formats for free as well Here's a tutorial on how to <a href="click here">convert wma to mp3</a>.
I use TuneCab. It converts any purchased or copied music or movie files and almost any audio or video file to MP3, AAC, WAV, MPEG4, DivX for CD, iPod and other MP3/MPEG4/DivX player file formats at high speed and CD quality - legally. click here