Logon time restrictions can be done with the net user
command built into Windows (xp and after as far as I know, but possibly also in 2000).
Open the command prompt (start > run > cmd , it is also usually available under the start menu) and type the following command
net user <username> /times:Monday-Friday,08:00-21:00
The above command will only allow the user with the specified username to login Monday-Friday from 8:00a.m. to 9:00p.m.
When entering the username do not include the <> and the times can be adjusted as you see fit. For instance you could do
net user <username> /times:Sunday-Saturday,12:00-13:00
That command will only allow the user to logon from Noon to 1:00p.m. every day of the week.
It is very adjustable, but can be a bit of a pain to get working properly. Remember, it only restricts the one username specified in the command, so if he has another user account it won't stop him.
Here is Microsoft's take on it, though their help is more confusing than it is helpful
edit: This command will not force the PC to logoff after the time expires, but will prevent them from logging back in.