Free tool to convert MP3 to WMA?

I have a Creative Zen Touch mp3 player which I have nearly filled with music. The tracks are a mixture of MP3 and WMA files. I find WMA to be as good sounding as MP3's but at a lower bit rate and thus they take up less space. Does anyone know of a free tool that will convert large batches of mp3s to wma at once? The only ones I can find will only convert one file at a time and obviously this is no good when trying to convert thousands of tracks.