How To Join AVI Files Without Reencoding Them?
I'm not sure if I get 100% what you want, so correct me if I'm wrong...

You can just use QuickTime (7 Pro or X -> I'm not sure about QT X). Just copy/paste the several files in one new window and export to .avi. It takes some time to export them, but you are not re-encoding them... 

EDIT:
Try downloading this plugin for playing Xvid-files with QuickTime.