If you attached lately a keyboard, a mouse or another USB-based
input device to your computer your computer may no longer shut
you could try the following procedure:
Right-click My Computer, click Properties, click the Hardware tab, and then click Device Manager.
Double-click Universal Serial Bus controllers to expand it, right-click USB Root Hub, and then click Properties.
Click the Power Management tab.
Click to clear the Allow the computer to turn off this device to save power check box, and then click OK.
If the above procedure won't work for ya, than More than likely, Windows has detected a process that is very
busy or stuck running high system resources. Press CTRL+SHIFT+ESC to
bring up the task manager. Click on the processes tab and terminate
any program that is stuck using 90% or more system resources, though
you should ignore the one called "SYSTEM IDLE PROCESS" . if that
fails, then proceed to terminate all processes under your USER
NAME or login. Next try to shut the computer down normally, and
if that fails press and hold down the power button until l the
system shuts it's self off
Good luck ...