How to keep track of computer time used?

Hi, I am in charge of a group of computers at a student lab and have just installed Samba as a PDC to authenticate Win98 clients. I need a way to keep track of the time that each student uses on each of the computer. I would like to have info on thetimeeach student logs on and at what time the student logs off, and sent to a centralized terminal. How do I go about doing it? Any ideas and help in this matter is deeply appreciated.