If there IS some sort of error message immediately prior to the shutdown
(such as "Remote Procedure error") then maybe you are being attacked by
another computer infected by a virus!
Dont worry too much. It doesn't necessarily mean that you have a virus
yourself..
Just run your "Windows Update" programme (from your start menu) and click on
"Scan for updates"
You will need to be connected to the internet.
Download all critical updates.
This should fix the problem and prevent your system from shutting down.
The only frustrating thing is that sometimes you might get the shutdown
error whilst downloading the updates!
Very annoying, but just keep trying.
I cant remember which specific update will fix the problem. Perhaps
somebody could post the update number and you can just select that one
first.
Brendan