Back-up…specific folders

I am looking for someone that can tell me the best way to back-up or sync a specific folder, with folders and files inside, on multiple hard drives.

Basically I have 2 computers with internal drives and one external drive I want this specific folder to be identical on each. I have Vista on one machine, XP pro on the other 2. I want to be able to hit one or 2 clicks and have it be done. I also DO NOT want to just copy over the folder everytime. I will be working on each computer on separate occassions and want to be able to do a sync, not over write and no deleting. Thanks in advance!!!