i have seen this only with a graphics card before, something was wrong in bios i think;

Closing networked apps and disabling the sil3114 doesn't necessarily prevent the cards from sending spurious interrupts. Try physically removing each card?

Things I've done so far:

* Removed Unifying receiver and disabled USB controller in Device Manager. No change.

* Unplugged the Ethernet cable. No change.

* Removed the Sil3114. No change, but the 5V rail is better now.

* Rebooted into Safe Mode. No change.

* Changed BIOS settings. (enabled PnP OS, MPS 1.4, and L2 ECC) No change.

Next up will be removing the Ethernet controller, and then changing the GPU. Slightly lost here. I have screenshots of my BIOS settings if needed.

It could be caused by many things. When I was doing tech support in late 90s, there was a similar situation with one win NT 4.0 pc and when I figured it out it was actually caused by a faulty keyboard.
I would start removing expansion cards and peripheral devices and checking out if removing any of them make a difference. If it is faulty hardware, disabling them in windows most likely won't make any difference.

Removed EtherLink XL, no change. Swapped GeForce 256 for TNT2, no change. I booted Linux, and it claimed there was a lot of interrupts coming from IRQ9, which is apparently for ACPI? This means it'd likely be coming from the board/firmware itself...

You don't have any sound card in the system?
I asked this because MPU-401 MIDI port also uses IRQ 9

Looks like there's a manufacturing flaw with the ASUS SMP boards that causes them to spew interrupts in ACPI OSes, especially Windows 2000. Either change to the MPS MP HAL or do the BIOS upgrade and resistor change (or just a solder blob) as mentioned in the ASUS QA article.

@Baoran: No, I don't.

You can disable ACPI in windows 2000, but unfortunately that disables SMP support too.

This brings back memories. I had a similar problem with an Exsys EX-3305 RAID card in my XP system years ago. Task Manager showed that DPC calls were consuming about half CPU resources. I never did solve the problem. I eventually swapped the Exsys card for a HighPoint with the same chipset.

The guide is incorrect - you can change to the MPS Multiprocessor HAL instead. This is an ACPI-free MP HAL, and it seems to work fine. After letting it reinstall the drivers, there's no more spurious interrupts. (It seems that FltMgr isn't happy after the change, but it doesn't seem to affect anything. I'd like to fix it though.

I'm going to change the GPU back to the GeForce DDR or a 9600 PRO though - the Vanta is an utter slug running at 1080p.