The issue here is that .AVI isn't just one thing. Like .MP3 is an MP3 is a song. Done. but .avi is the defacto extension for...... at least five... maybe up to eight different video formats. It's confusing and troublesome and irritates people but there ya go.
Thankfully, once .avi started to mean every goddamn video format under the sun people started making packages to let computers play all this crap back with just a single install. I'm fairly certain the best at the moment is called the Combined Community Codec Pack or CCCP, and you can get it at
http://www.cccp-project.net/ What that does is install the specific filters needed to play all these video formats that all end with .avi Once that's installed Windows Media Player or Winamp 5 or VLC or Zoomplayer or whatever will be able to play basically everything under the sun.
There's another video format that I'm fairly certain CCCP installs support and a specific player for called .mkv, and that's used primarily for rips of foreign language films, and other things with multiple languages and subtitles like anime, so you'll be able to play those back as well if you run into them.
