XP Pro and Vista Ultimate 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?

 

[Andrew E.]

99% of the time,when a pc does that it usually has a hardware conflict
somewhere.Try removing add-ons one at a time till the conflict ends.

 

[Polaris]

I replaced power supply. Normal shutdown working normally so far.