It might just be the compression codecs those particular AVIs are encoded in. I just realized today that the videos my digital camera captures (AVIs) can't be imported into Adobe Premiere for that reason. You might try updating your DivX, XviD, etc. codecs, but if you're sure they're pretty up to date, it might be a quirk in Movie Maker. You never know with Microsoft stuff (i.e. expired use time).