The file you are trying to watch probably uses one of those stupd rare codecs....I've read that no matter what quicktime avi plugins usually dont work.
I know this might come off as a shock, but if worse comes to worse, you could always try *Gasp* Windows Media Player for OSX
http://www.microsoft.com/windows/win...x/default.aspx
The other alternative ive read about is a program called VLC, that is aparently pretty good. The bad news is that aparently they stopped development of it The only problem so far with that is some of the newer codecs wont be recognized by the program and never will.
Hope this helps.