You may also be able to play the files by converting them to mpeg format using DiVX Doctor. This is especially effective if you can play the file but you can't hear the sound.

In OSX occasionally I have to run DiVX Doctor from Classic (OS 9) just so I can allocate it more memory to get through a really big file. I think DiVX Doctor is available for OS9 and OSX, but I'm not sure.

I really wish people would share mpegs instead of avi's. Oh well, at least avi's beat Windows Media Player format. I have never come across a WMP file that didn't suck.