How to play avi files?

I've downloaded several videos from a filesharing software, but I can't seem to play the avi files. The flv file won't seem to have any problem. But many of my videos are avi formats and I need to find a solution here. None of the players (Windows Media Player, PowerDVD, Ares etc.) in my computer can play the avi files. I'm sure there's some sort of a missing codec. If you know what it is? How I can get it? Or is there any other solution?