Vista Ultimate and XP Pro shutdown and immediately restart

[Polaris]

System Configuration:
HD1 - XP Pro with all updates
HD2 - Vista Ultimate with all updates
I switch OS at boot time via F10 key.
Problem:
I select Turn Off computer and system executes normal shutdown procedure. A
second or two after shutting down it Restarts. If I press the Power Btn. and
hold it down until the system shuts down, power remains off. This occurs
using either OS so I suspect something common to both; perhaps the power
supply? Any suggestions?

 

[Mark L. Ferguson]

Look in the Event Viewer. (Click start, type eventvwr.msc) The error is
going to have an entry at each shutdown. You will probably be able to use
the Properties of the file mentioned in the error to identify the failing
device (that causes this 'restart or error')

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Mark L. Ferguson MS-MVP
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[Polaris]

Thank you both for replying. I replaced the power supply and system is now
shutting down without restarting.