Sometimes you just need to convert .mp4s to .avis, right? The problem is then finding a reliable program through which you can do that. Once more, most of what is available is going to cost you.
I found this website, however, where you can convert all for free and the output is of reasonable quality. It is probably not acceptable for most of what you want to do, but if you are going to use the movie in a Power Point presentation, for instance, it should be something.