Sometimes windows will shutdown for "updates", which is great and all except when you had an unsaved project in the works or a huge file conversion that you expected to be completed when you wake up, only to find your computer rebooted.
Check out the links below for instructions and download links.
Windows Update's automatic reboot can be one of the most annoying "features" in Microsoft operating systems. While it's a bit less pushy with Windows Vista and 7, allowing users to postpone the restart for up to four hours, you may not be at the computer to intercept the first or subsequent notifications. Prevent Windows Update From Automatically Restarting Your PC.