I'm affraid it will cost you. There is no standard way of doing this. You have to use CConnect.exe, which is included free in the nice pricey Windows 2000 Server resource kit. I don't know if you can get it from anywhere else, a quick google, just points me to articles about it in only being available in the Win 2k Serv resource kit.
It is pretty complex configuration of the tool, and it needs to be installed on each machine. However, you can distribute it through group policy, or logon scripts. You just need to plant it.
It basically works by registering a session in the database, when another one logs on, if a session for that User is active, then it logs the last logon machine off.
There is a third party tool available aswell, I think called UserLocker. However, I don't know if it still works in post NT domains.
Also, I've not heard of any free way of doing this, as it is not client side controled, therefore can't be processes solely through Group Policy, as no mechanism is inserted into AD to check for concurrent logons.
Hope this Helps.
Neil J Cotton
njc Information Systems
Systems Consultant