Windows restarts without user prompt when certain updates has been installed automatically during Windows Update leaving the following message:
Your computer was recently updated! Windows recently downloaded and installed an important security update to help protect your computer. This update required an automatic restart of your computer.
- Go to Start > Run and type gpedit.msc
- Under Computer Configuration go to Administrative Templates > Windows Components > Windows Update
- Find No auto-restart for scheduled Automatic Updates installations, double click on it, Enable it and click OK.
More information (server auto reboot policy change): http://blogs.technet.com/b/mu/archive/2008/10/02/windows-update-and-automatic-reboots.aspx