Computer doesn't switch off automatically.

I have a computer running Windows XP Home.
Until recently to switch this off I'd START Turn Off Computer
Turn Off. This would then tell me "Saving your settings" followed by
"Windows is shutting down". The computer would then switch off
automatically.
Then, inexplicably, it stopped switching off automatically but just
hung at "Windows is shutting down" - leaving me to hold down the POWER
button to switch off the computer.
Thinking this was a problem within my Windows I re-installed it
expecting that to resolve the matter - but again it hangs at "Windows is
shutting down".
What must I do to get it to shut down automatically again? My other
computer, also running XP Home, shuts down automatically as I wish this
one to do.

TIA of any replies.

--
DB.