Do you want to restrict each user so that they can only log onto their own
individual workstation?
In the Property sheet for the user object, click on the Account tab, and
then "Log onto". This setting is creating on a per-user basis, not through
AD/Group Policy. (Though I imagine it could be scripted.)
If you're looking to limit users to one concurrent logon (but still allow
them to log onto multiple machines), you can also look at the cconnect.exe
utility that's a part of the Windows 2000 Resource Kit. (It's unfortunately
not one of the free tools, you need to buy the print copy of the RK and it's
on the companion CD.)
http://www.tburke.net/info/reskittools/topics/cconnect.htm