P4C800 Deluxe fails to initialize

I have a PC based on a P4C800 Deluxe motherboard that has been running very reliably since Aug 2003. Last week, after closing Internet Explorer, the only program running at the time, the computer froze up. No mouse pointer movement, no response to Num Lock or Cap Lock on the keyboard lights, no evident way to reboot. No BSOD or any other errors indicated. I shut it off with the power button, and it's never started since.

Upon power-on, the power light and the hard drive light come on, but the network detected light at the cable connection does not. The main drive spins for about 4 seconds and goes silent. No signal is detected by the monitor, much less any display that might indicate the problem. The motherboard makes no beaps.

I've replaced the video card with a known good AGP video card. I've reseated the original video card, a Radeon 9800 AGP. I've reseated the RAM cards, two of them for a total of 1 GB. At all three steps, there was no change in the machine's behavior.

I've inspected the motherboard for any capacitors that have popped their top, or other evident problems. I don't see any problems, and all connections seem secure. The computer does not get moved or bounced, it just stopped.

Suggestions? It doesn't even initialize to the point of looking for boot media, diagnostic software will be hard to run. There have been no recent hardware or software changes to the machine.

Eager for ideas,
Peyton