I need help with .avi files and QuickTime

I want to play .avi files on QuickTime pro. I could do it on my old computer and swear everything's in the same place on this one, but it's not working. Can someone please tell me exactly what extension(s) I need to have and where I need to put them?

Thanks

I Got An Answer!

Haha, I posted this and only after read trpnmonkey41's response to stevo-m's post. That (and Perian) solved my problem. Big thanks to trpnmonkey41 and stevo-m!!!