Quicktime Player should be able to play AVI files, but some codecs seem to be unsupported and require extra extensions. Try getting the Intel Indeo Video codecs (search on asu.info.apple.com). It could also be compressed using DivX, for which you'll need the WMP for Mac/DivX player combo. There is also another extension I know of called "Windows Compressors", but I can't find the exact lovation to download it from, should be on a Microsoft server somewhere.