There are several possibilities, including a virus, proocess that cannot/will not go down, or a simple glitch with Windows. (Windows is not invulnerable to such things.) Press and hold the power button for 7 seconds and it should turn off. If not, pull the power cord and plug it back in. When you turn it back on it may give you an alert that an invalid shutdown was detected...just continue on as usual. Once booted check the following:
* Run a virus and spyware scan again, just to be on the safe side.
* Check Task Manager (ctrl + alt + delete)and MSConfig (start->run->msconfig) for unwanted/unnecessary programs.
* Check Event Viewer (start->control panel->(performance and maintenance)->administrative tools->event viewer) for errors that may have come up just before the computer began shutting down. (Double-click "System" and look for the red flags.)
* Try shutting down again and see if it repeats itself. If not it was likely a one-time (well, with Microsoft there's no such thing as one-time, but...) glitch.
Hope this helps,
John