This behavior may occur if Windows stops responding during a typical operation or during the shutdown process. By default, the computer is configured to automatically restart when Windows stops responding. To view this setting, follow these steps:
Click the Start button, right-click My Computer, click Properties, click the Advanced tab, and then click Settings under Startup and Recovery.
Under System Failure, view the Automatically restart check box. If the Automatically restart check box is selected, Windows automatically restarts if the computer stops unexpectedly.