Why won't my GX150 shutdown?

I have a Dell Optiplex GX150 running Windows 2000 Pro with all available service packs and patches.  The machine works wonderfully but the problem I'm having now is that when I try to shut it down it just restarts.  If you press and release the power button the same thing happens.  The only way to get the machine to shut down completely is to unplug it or hold down the power button.  Any help would be greatly appreciated.
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Take a look at the solutions itemized here: http://www.aumha.org/win5/a/shtdwnxp.htm
GTKINCAuthor Commented:
I'm running Windows 2000 Pro.  Not XP.  Also I personally feel this is Hardware.
If you remove all the gui features of WinXP, it will resemble Win2K.  There are also many drivers that work on either WinXP or Win2K.

You haven't explained why you think it is hardware; perhaps more information would reveal that to be really the case.  If it is a hardware problem, I would look at the power supply, but that is just a guess based on what you've stated so far.
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I would reinstall SP4. this usually fixes all kinds of issues we have with 2k.

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GTKINCAuthor Commented:
I say this becuase when you physically push the power button and Windows says "shutting down" it goes through the motions of shutting down but once you think it has it is actually restarting.  The same thing happens when you choose shutdown from the start menu.  This is the only reason why I said that.  
They both trigger the shutdown sequence, but that does not restrict it to a hardware problem.  Have you checked the BIOS for any wake-on-activity settings, such as LAN, keyboard, or modem?  You should check the link I posted in the "REBOOT INSTEAD OF SHUTDOWN" section.
Backing up Callandor..
That kind of shutdown is a software shutdown and is controled by a circuit on the motherboard.
It's not like cutting power to the sysytem as it was with older computers.
The same circuit controls starting it up and there are several 'triggers'.
Yes it could be hardware but it is more likely it's a setting.
One non-hardware senario would be:
If you have your BIOS set to start up the computer on LAN activity.... (Wake on LAN)
.. You shutdown .. But the PC senses some LAN event and it starts back up again.

There are also some virus's and malware that won't let you turn the computer off.
Update your spyware and antivirus.
Also get all the updates for Win2000 from Microsoft.

The site given by callandor is excellent. The most likely culprit is roxio (been there, done that, use something else now).

Clear the "restart on error" check/btn, as what is likely to be happening is that the system is having problems closing an application or service as windows is shutting down, windows is seeing this as an error, and hence restarting.

GTKINCAuthor Commented:
Don't know why but I reinstalled SP4 and that fixed it.  Thanks to all for your assistance.  Also thanks for the information on hardware vs. software.  Very useful.
because the service pack resets config and restores core os files.

Glad it worked for ya!
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