Interrupt Windows 2000 execution

[loganmex]

I'm working for a company that is renting computers, and they need a system that counts the time the user is in the computer. When time is up, the user gets unable to use the computer, via blocking Windows or some programs. Once the user pays again, we should be avaible to restore Windows, and re-start the counting. Well, that's the general idea.
Is there any sugesttions of how can be done, code I could use or where to look for info?

Thanks