There's a piece of free software called "ffmpeg" which will do all sorts of conversions, including bitrate conversions for you. It's also built into a number of other software tools. Unfortunately, I don't know how readily it'll run under Windows - most of the software packages I've seen are built for Linux or the Mac, and I've seen anecdotal reports of not being able to get all the codec libraries on Windows. You can search around, and maybe you'll be in luck.
On my Linux machine, for example, I would do one of these commands, depending on which version of an ".avi" file I wanted as an output, with a video rate of 200kb/s:
ffmpeg -i <inputfile.ext> -vcodec msmpeg4 -vtag MP43 -acodec mp3 -b 200 <file.avi>
ffmpeg -i <inputfile.ext> -vcodec msmpeg4v2 -acodec mp3 -b 200 <file.avi>