User Login / Logout timer

Running MSServer 2003 with active directory.
+- 8000 users (in groups), +- 400 PC’s (Win2000)
How do I create a time limit of 1 hour per user, per day? If the user Logs out after 30 min and Logs in again on a different PC, the remaining 30 min must remain. Does anybody know of a way to do this? Or even “Timer Software” that can handle my needs and must be able to work with Active Directory.

Any comments will be appreciated.