Annoying Windows 2000 "shutdown"

Greetings!

I've been trying to track down the source of this problem on my HTPC/DVRfor over a month now, and I'm going nuts! Hopefully someone out there will have some insight.

Here are my computer specs:

AMD ASUS A7N8X-E Deluxe Motherboard; Athlon XP 2600+ processor; 1GB RAM; eVGA 128MB Geforce 6200; 1x250GB IDE HDD (apps, photos, music), 1xSeagate 300GB SATA HDD (Recordings); Antec Sonata Case; Windows 2000 Pro SP4; SageTV v6.0.19.120; 1xHauppauge WinTV-PVR-350 Capture Card

The problem is a random "shutdown" of the computer. Sometimes it shuts down during the boot sequence, sometimes it shuts down when Windows is loading, sometimes it will go for hours or days until it shuts down. When it does shut down, the case fan and CPU fan are still operating, the computer is still on, the hard drives are still powered up, but the monitor goes dark (no signal), and nothing else works (mouse and keyboard don't respond, etc.). In one case, I was listening to an internet radio broadcast through my stereo speakers when it shut down, but the broadcast continued to play through the speakers!

When I reboot the computer, and Windows loads successfully, I occasionally get the Restore Active Desktop/unexpected error screen on the desktop. And I have had numerous instances of Event ID 51 in the Event Viewer, stating:

"An error was detected on device \Device\Harddisk1\DR1 during a paging operation. "
That leads me to believe it's a hard drive issue...

Here's what I've done to troubleshoot:

-I've reformatted both hard drives and reloaded the OS
-I've blown out the case of all the dust bunnies,
-I've run the Windows RAM diagnostic utility for hours, with no failures,
-I've run Spinrite on both hard drives, with no failures,
-Virus scanning and spyware checks are negative,
-Graphics drivers are current,
-Checked connections (hard drive cables, RAM seating, etc.) and all look in order,
-Checked CMOS battery voltage, and that's OK (around 3.2 V last time I checked)
-The CPU and MB temps look reasonable (CPU is around 51*C).

This problem occurred occasionally prior to reloading the OS, so I can't believe it's a software-related issue, but maybe I'm missing something...

I'm plum out of ideas and really frustrated! Any help is most appreciated!

Thanks in advance.

Definately sounds like a hard drive issue to me... see if you can scan it with the manufacturer's diagnostics.