It sounds like you would have to run this script as an NT service in order to get it to run when no one is logged in.

Update: I just tested this in Windows 2000, you can set a scheduled task to run any time the computer is on by first logging in as administrator and then making the task from there. It will ask for a user and password to run the script as. This will ensure that the script runs even if no one is logged into the machine.

-caedes