You must have "Automatic Updates" enabled on your system. One of, or all of the updates must have needed the computer to be restarted to complete your system update. If you would like to turn this feature off, here's how:
Go to Start>> Control Panel>> System>> Automatic Updates tab. Here you will be given other options for downloading the updates.
-or alternately-
Press the Windows key and then Pause/Break on your keyboard to retrieve the "System" applet.