Playing .avi format files in QuickTime 7

I've got a few .avi format files on my computer, but when I try to play them, they don't work and a message comes up saying I might need to download some plug-in's or something. Anyway, I found something call DivX 5 which will allow me to play these .avi files. Unfortunetly, it is not compatible with QuickTime 7.
So are there any other plug-in's I can download, or will I just not be able to play my .avi files?