You could try different codec packs. Some people have had problems with windows movie maker choking up on codecs. It's kind of odd, it's like it needs you to isntall thme so it works, and thne it wants you to uninstall them so that it can continue working. Makes absolutley no sense to me, but apaprently it has worked for some people. In my opinion, WMM is pretty nonsensical, so I guess nonsensical things must be combated with nonsensical methods?
Anyway, I would stick with AVS for now. If you get tired of that you could always try Virtual Dub or many of the other alternative video editors.