I agree VirtualDub is the best program for joining divx .avi files. It is also one of a few programs that can keep the audio in perfect sync with the video, after joining. In a lot of other programs I've tried, the audio becomes out of sync with the video when the joined .avi is played.
Here's the guide for using VirtualDub to cut/join divx .avis.
CODE
http://www.dvdrhelp.com/virtualdubedit.htm