avi, like quicktime, is a container format that can hold lots of different types of video. There are some that you just plain won't be able to play. But lots of avis these days are divx/mpeg4 and there is a quicktime plug in for that -
http://www.divx.com/
You should also get divx doctor while you're there - it can fix some flaky avis.
MPEG Streamclip can if time code breaks on mpegs, I don't know where to get it.
Also get the apps mentioned above. They will play lots of stuff quicktime can't.
So what kind of 'media files' are you trying to watch?
