I have an older D865GVHZ board that I've been playing around with. It has a 3.4 ghz P4 and 2 gigs RAM. I recently added a PCI 8400 GS 512 mb Nvidia video card. For some reason when I shut the PC down it just restarts when that card is installed. When I remove it, it shuts down normally. I have "Plug & Play" disabled in the BIOS and have tried it enabled with no change. Maybe it's just an issue with the card. I don't have another PCI card to try and the board doesn't have an APG slot. It's not that big of a deal and it runs great but it is annoying to have to press the power button again when the Intel splash screen appears to fully shut it down.
The O.S. is probably crashing at shutdown, and you have the option to reboot after a crash. That's the default. If you change that, and I recommend it, instead of rebooting, it will sit there showing you the blue screen of death. I'm guessing the video driver is bad.
To change the option, right click on "My computer". Click Properties > Advanced tab > Startup and Recovery Settings. Uncheck the box for Automatically restart.
Actually it's not blue screening. It goes through the normal shutdown process, "Saving Your Settings" then "Windows Is Shutting Down". The drivers are Nvidia's latest. It's just like you select "Restart" in the shut down menu instead of "Shutdown". Windows event wiewer is clean also. When logged on to Windows, the computer runs great and I can even watch Blue Ray movies with my Blue Ray drive so I'm not really thinking it is a driver issue or Windows issue. Something about this board or some setting in the BIOS or possibly the card itself is the culprit. I was hoping someone here had seen this problem before and would have a suggestion. By the way the OS is XP Pro.
Power supply is a good place to start.
Some power supplys have issue with large current draws at shutdown causing the standy voltage to dip as the system is shutting down.
This dip is interpited by the mother board as an AC power failure and depending on how the BIOS is configures, the board powers-on after an AC failure.
If the board you have has options in BIOS to stay off after AC failure, it will likely stop the rebooting.
I had the same problem with a PCI 8400GS that I had purchased for a customers old Dell PC (865 chipset if IIRC) so that they could run a new widescreen monitor.
I tested this on a HP DX2000 MT, I tried a few different settings in the Power Management options but the olny one that actually resolved the issue was setting the 'Remsume on PME' to disabled, after that when you shutdown the PC it will not restart just shutdown as normal.
I hope this helps somebody.