mountain lion wont play .avi files

QuickTime 10 does NOT support these file/containers natively.

You need to try something like VLC media player or MplayerX

VLC and MplayerX are in the MAC App store or can be downloaded from these links

http://www.videolan.org/vlc/download-macosx.html

http://mplayerx.org

The old Perian codec plugin bundle is 32bit and no longer supported by OS or the project development team !

Some codec types are already broken AC3 so I no longer recommend it.

An interesting read on tricks to get QT to play AVI, Try this site for info on dot avi files:

http://www.video-converter-mac.org/play-avi-with-quicktime-on-mac/