Two things.
First, get a better player for avi files then WMP. There is a free player called BS Player. Here is the link: Make sure you grab the free one.
Second, the link Lilpinkflower gave you is a great tool. Download that, which is free also, and use it to open the avi file. It will then tell you which codecs are needed to play that file and whether or not you have it installed on your PC. Then google each codec and install it. Then it should work fine.
