Windows Vista: Vista Ultimate 32-bit Won't Shutdown! Maddening!

Vista Ultimate 32-bit Won't Shutdown! Maddening!

I installed Vista Ultimate 32-bit as a clean install on my system. When I attempt to shutdown or restart my system will go through the standard shutdown process, but fails to kill power. The screen will go black (and the monitors will even lose signal), but the fans and drives and such continue to spin. I do not have any driver conflicts (or at least none that Vista informs me of) and I've tried every possible power configuration I can think of (in BIOS and in Vista).

My System:
1000W Power Supply
EVGA NVIDIA nForce 680i SLI Motherboard (revised quad-core edition)
QX6700 "Quad Core" Core 2 Extreme Edition overclocked to 3.2 GHZ
2GB Corsair XMS2-8500C5 DDR2-1066 RAM
2 X Western Digital Raptor Enterprise 150GB 10K RPM (RAID 0)
Seagate Barracuda 750GB 7200 RPM 16MB
All-in-One Internal USB 2.0 Flash Card Reader & Writer
20X Dual Layer DVD±RW Drive w/ LightScribe Technology
2x NVIDIA GeForce 8800GTX in SLI
Creative Labs Sound Blaster X-Fi™ Fatal1ty FPS
Microsoft Windows Vista Ultimate (32-Bit)