Limiting time online

Can anyone tell me how I can limit the time users of my network spend online?

Using XP pro I have three computers sharing an ADSL connection via a router. How can I gain some control over what the other computers on the network do?

Ideally I would like to restrict the time users can stay online as well as view an activity log.

I know it is a big subject and difficult to answer in an e-mail so maybe someone can give me a clue as to where to start or send me a link to some reference on the web. I don't mind doing a bit of reading.

Thanks in advance for any help I my get.