Here is a strange one...

Over the past couple of weeks my XP constantly refuses to close down correctly after being in use for a number of hours.
I press the Start Button to gain access to the "Shut Down" option window.
Pressing the pop-up "Shut Down" causes the computer to freeze with a blank screen, (no Icons).
Only the mouse pointer responds when the mouse is moved.
The "Task Manager" can be accessed with Ctr+Alt+Delete but any attempt to close the computer down from there by any method results in a complete freeze of everything, and the computer can only then be shut down from the Power button, or restarted via the Reset button, after which the computer can then be shut down normally by the usual method.
Note: The Task Manager shows no unusual program activity being performed by the Processor....(all Processes are zero activity, apart from any mouse movement).
A virus scan shows nothing unusual, and Spybot shows no mail-ware, spy-ware, etc.
The only thing I can think of, is that something important has changed in the Windows Main Directory, preventing a proper shut-down procedure being accomplished from the Start Button.
Any ideas, Chaps...?
Paul... 8-)...!