Never tried this, but here is a c&p:
* Start the Task Scheduler. Under Windows 2000/XP this is located in Start Menu > Programs > Accessories > System Tools.
* Check that the Task Scheduler is running. You may do this by checking the Advances menu and seeing if it lists 'Stop Using Task Scheduler'. If it is listed, this means the Task Scheduler is running. If it says 'Start Using Task Scheduler' then the Task Scheduler is not running. Click on the option to start it.
* Now create a new task by selecting the File menu and then the sub-option New and then the sub-sub-option Scheduled Task. Rename the new task something like Reboot. Double click on the new task
* First lets set the time. Click on the tab Schedule and select Weekly for 'Schedule Task'. Now select the day and time. Normally a weekend day (Sunday say) and a time at night (say 1am) to ensure no-one is using the system at the time.
* Now to set what is to run. Click back on the tab Task. Click set password and enter the password you use to log-on to Windows. This will ensure the reboot happens even if you are not logged on at the time.
* In the field 'Run' you now enter:
For Windows 98/ME - RUNONCE.EXE -q
For Windows 2000/XP/2003 Server - SHUTDOWN.EXE -r -f -t 01
you may also need to download this file: http://www.nch.com.au/kb/attachments/10001.zip