XP Pro
Well it depends on whether the computer in question is a stand-alone (doesn't require a login by the user) or is managed and each user has an account that requires them to login. If they are required to login then you can set limits:
Click Start, Run and enter GPEDIT.MSC Go to Computer Configuration, Windows Settings, Security Settings, Local Policies, Security Options. In the right pane, locate the entry for Network Security: Force logoff when logon hours expire
More information on setting logon hours can be found in Help and Support. Type "logon hours" without the quotes in the Search box. Go to Full text matches and click on the Net user entry.
Examples of setting the logon hours would be:
net user johnsw /time:M-F,08:00-17:00
net user johnsw /time:M-F,8am-5pm
net user marysl /time:M,4am-5pm;T,1pm-3pm;W-F,8:00-17:00
If it's stand-alone, or just for an overall easier method, you may want to consider third party software that will do this for you. A good one is WatchDog, and they even offer a free 30 day trial.
http://www.sarna.net/watchdog/
~ Jerry