PC Keeps 'Shutting Down'

My PC keeps shutting down about every two minutes, even when I am in
the middle of using it. When I hit the power button it comes back up
in exactly the same place it left off, or after a minute of so it will
come back up on its own.

Until yesterday it was going into hibernation in the same way so I went
into the Power Options program in the Control Panel and disabled the
hibernation mode.

I think it started after some MS automatic updates last week.

Is there a System Rollback-type option for Win 2000 like there is for
XP (and if so, how would I get to it), or does anyone have any other
suggestions please?

Thanks.

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