For the mac VLC is a much better or reliable previewer than QT!!! http://www.videolan.org/vlc/ (choose nearest mirror site) This is what VLC can play: http://www.videolan.org/vlc/features.html

QT requires extra plug-ins to play many of the formats.
This 'always' happens with all avi files??? 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. If you don't already have it, dwnld divx & 3ivx. I'm not sure you'd be able to use them for encoding if you're not a QT Pro user but they should help decode your movies. 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/
ALso see: Setting & updating QT MIME Plug-ins