Windows 7 automatically reboots my machine twice in a week after it finished the windows update. This might be alright for some people but I found it very annoying, so here comes the remedy:

1. Run gpedit.msc
2. Click Administrative Templates under Computer Configuration
3. Click to expand Windows Components
4. Go to Windows Update and enable No auto-restart with logged on users for scheduled automatic update installations

See also: Disable Windows XP from auto restarting after windows update.