Clarification: If all you desire to do is enable playback of .avi files, there is a simple way that involves only one of the above tools, Divx, and its associated player.
First, go to:
http://www.divx.com/divx/play/download/ That is the download page for the free Divx codec and player.
Don't enter your email and uncheck both boxes next to "Yes, I would like to get updates about new DivX versions and keep up with what's hot in the world of DivX." and "I'd also like to receive special offers from DivX partners."
Make sure only the "DivX Play Bundle only (Windows 2K/XP)" is selected under "Installer" and then click "Download." Save the file someplace you can get at it. Run the file once it is downloaded.
Once Divx and the player are installed, try opening your .avi files. They SHOULD open in the Divx player. If not, go to Start>Programs and find the Divx player (Should be under Divx something) and open the player from there. Then use whatever open dialog the player has (It is probably something like File>Open) and find the file you want to play. Select and open. It should then play, provided Divx was the original problem.