Is there a simple tool to down convert the quality of mp3s for size purposes?

I am trying to put everything I have and want on my Zune and when I rip CDs, I rip them all at 128, but some of the holes in my collection, I’ve been downloading, and most of the downloads are at higher bit rates. Let’s say I downloaded 200 albums at an average of double the 128 bit rate, well, that means I’ve just robbed myself of space for 200 albums. So what I’m looking for is a program, preferably a free one, where I can specify the folders (I have one folder per artist and a sub folder per album) I would like to convert all MP3s in that folder from whatever the bit rate is to 128. Is there such a thing? I don’t want something where I have to do it one file at a time, that would take forever. And I can’t really afford to buy anything pricey to do it, if I end up getting to the end of my collection and start running out of room I’d rather delete a few things than have to convert each individual file to free up space. Any suggestions?