Go to your QT Prefs>Plugins>MIME Settings (at bottom of window is a mime button) In the new window tick ALL options for plugins. See Setting & updating QT MIME Plug-ins If you don't already have it, dwnld divx & perhaps 3ivx. AVI is very much like QT. It has a generic video format, yet it can also be a hybrid format incorporating other types of compressed formats. Becoming very popular is avi-divx. For DivX go to http://www.divxmovies.com/codec/
For 3ivx go to http://www.3ivx.com/ & go to dwnld page & look on top right of page.
I should also have mentioned Xvid which you'll find at the divx site I gave you.
There's also other players you could try such as VLC: http://www.videolan.org/vlc/ or MPlayer: http://mplayerosx.sourceforge.net/