use mstsc and then logon. That will prompt if a person is still logged on. if you select to continue, it will log them off and allow you to log on.
Also, shutdown.exe, which I think is installed with XP of from the NT4 admin kit,
shutdown.exe -r -m \\computername -t 00
that will reboot the machine remotely. You can't use that to log someone off, as it's only designed for local use.
There is another method using WMI, but I can't recall off hand.
hope that helps.