System will Shutdown but not Restart

Hello all,

My Dimension 8250 has developed a bad habit in which it will not respond to Restart commands from the OS (Windows XP SP1) although it does still Shutdown properly. When asked to Restart, Windows will proceed normally through the process but stay forever on the "Shutting down your system ... " screen. I have to hold down the power button for several seconds at this point to get it to shut off.

It's not an OS problem, I've reformatted twice since it began. Something is either damaged or disconnected, I'm sure.

I've built computers for the past 11 years, and my first thought is the power supply has a problem of some sort, but I know how expensive (and proprietary) Dell's power supplies are. I am hoping there might be some other, less expensive problem I've simply overlooked.

Yes, I've had the computer open several times. I've added a second HD and upgraded the video card to an ATI Radeon 9800 Pro (which requires a power connection). I also added a second bank of Dell-direct RAMBUS memory. Other than that, the guts are stock.

I've also been through the BIOS a few times and configured things to my tastes, and don't remember the settings I adjusted ever having an effect on reboots in the past.

Thanks for any help from "the pros" that hang out here!