Check QuickTime codecs, download latest divx codecs, try VLC, MPlayer, Flip4Mac... Barring that my only suggestion is to verify the avi file isn't corrupt, and if it is not, find out what codec you need to use to watch it. Now on my PC I have an app called GSpot which does exactly that - it analyzes a file, tells you if it's corrupt, what video codec, audio codec, etc,. you need, where to download it, etc. - but I'm not aware what the OS X equivalent is for it. Anyone?
Perhaps search on VersionTacker for something that might assist you...