most of those .avi files are probably DivX files. you COULD go here....
www.divx.com and download the codec to play on quicktime, OR you can download Mplayer or VLC. I recommend VLC because it plays ALMOST all movie/sound files. Go here to take a look...
http://www.videolan.org/
Oh, and if youre trying to play WMV3 files, only micro**** Windows Media Player 9 will play those. If the movie is .avi and is compressed with WMV3.... well you're SOL, VLC cant read WMV3, and the WMP cant even view .avi files.