I can heartily echo the recommendation of VLC, but I just thought I'd contribute some more general information to the thread too.
AVI files are somewhat odd beasts because it's a very meaninless file extension that says very little about the content of the file itself. There is no AVI codec, AVI is just a so-called container file. It holds raw data (video and audio files), and the metadata that descrives the codec that is needed to play the content. This means that an AVI file can contain any codec under the sun, so you can easily end up in the situation where some AVIs will play, and other won't, simply because you have some codecs, but not others.
The reason I love VLC is that it comes bundled with a massive codec library, so you just install VLC and never have to worry about codecs again (in theory anyway).
One of the most dangerous things you can do is google for codecs and download and install what ever google sends your way - fake codec packs are a VERY popular way of spreading viruses and spyware. Best to avoid the codec hunt altogether by simply installing VLC and using that!
B.