Well, there are quite a few, most popular I'd say would be DivX player, also VLC player and Media Player Classic. You should also try some codec packs as I am assuming you use windows media player, so try going to http://www.free-codecs.com, you should find some packs there...
I'm wondering where Val is, she always has very useful links and tips for this sort of thing 😀😀😀
Can't play AVI's, eh? Sounds like a Divx issue, as Janus noted. Either grab the free Divx codec and player at www.divx.com or get another player that comes with Divx support. I recommend Media Player Classic if all you need is to play AVI files. NOTE: Windows Media Player sees all AVI files as its own domain, but it can only play some (And the ones it cannot play include most DIvx media). If you try to open the files and find Windows Media Player trying to view them, manually open Media Player Classic or the Divx Player and open the file from within the program.