Download and install the Indeo codecs from http://asu.info.apple.com
You can't just say "can they play avi's" cause AVI's aren't always the same.
'AVI' isn't a format, just a wrapper.
It might have AC3 sound and Indeo video 5 inside it.
So you'll need those two codecs to see and hear it.
You get what I mean.