Control when a user can use the computer

You can use the net user command at the command line to restrict when a user can log on. Of course, you will need to have the user name and password and you'll need to set accounts and passwords for everyone else that this user doesn't know, so they can't simply log on as someone else.
For example, to allow this person to only log on between 8 a.m. and 8 p.m. on Monday through Friday, you would type "one of the following" at the command prompt:

C:\>net user harry /time:M-F,08:00-20:00
C:\>net user harry /time:M-F,8am-8pm

To get to the command prompt window, in the Start | Run box, type: cmd.exe

This keeps them from logging on after 8 p.m., but if there already logged on, it doesn't force them to log off.