VirtualDubMod should be able to do it (I hesistated to recommend it before others had spoken up because it's really general-purpose video-editing software, and can be a bit cranky--it's used fairly commonly for creating fansubs). If you can't get at its homepage, there are plenty of free software sites around the Net that distribute it--run a search. There's a users' forum at http://forums.virtualdub.org/?c=5 that may be able to help you get it up and running--my advice would be less than useful, because I haven't used it in a couple of years.
If you're really, REALLY impoverished and desperate, there's always mencoder and/or ffmpeg, which are difficult to use but can convert just about anything to anything.
http://www.videohelp.com has a massive tools-and-converters list that may help you find something more to your liking.