Assuming you are running windows it is best to have the latest version of Windows Media player or some other multimedia program. This will allow you to watch .avi files. However there are different formats for .avi files so you will need to have the correct codec installed on your system to allow you to watch it. There are a number of codec packs available but I can't think of any off of the top of my head. Do a search on google and that should bring up a number of them. So long as you have a player and the correct codec installed you should be able to view the file.
Try unisntalling Windows Media Player first, they download the most recent version from Microsoft. If that doesn't work, you can always download DIVX DIVX