looks a little crude but should do what you're wanting. Looks like it only does MOV files to mpegs, but the page that linked to it seems to think it'll work with avi's too
You could just use Quicktime pro, assuming the avi will run, if not then Cleaner 6 will do the job, and it renders mpeg better than anything else short of broadcast hardware, but it's SSSSLLLLLOOOOOWWWW