Again?
Man, this question comes up a lot, and I'm surprised by the number of responses that are completely uninformed. Doesn't anyone google the subject or look it up on wikipedia before posting here?
First and foremost, .avi is a *container* format, just like .mov is Apple's container format. It has *nothing* to do with the codec used to produce the content in the container. Telling someone to use flip4mac, simply because he has a .avi file is nonsense. Flip4mac is only suitable if the contents of the .avi file are wmv/wma encoded. If the contents of the .avi were encoded with xvid/divx, the flip4mac isn't going to do squat for you. Conversely, Perian will not help if the contents are wmv/wma, but will play xvid/divx.
So, the first thing to ask when someone asks about playing a .avi file is: What codec was used to encode the content of said .avi file? Only then can you answer the question correctly.