Windows Server 2003
Windows Server 2003 comes with the built-in ability to schedule your server to reboot at any time of your choosing. It is quite simple to set up.
Navigate to Start -> All Programs -> Accessories -> System Tools -> Scheduled Tasks.
Click on the icon which says Add Scheduled Task.
You should now see the Add Scheduled Task Wizard. Click Next.
Disregard the list of programs, and click the Browse button.