W2k3 BSOD IRQL_NOT_LESS_OR_EQUAL

I'm getting BSOD on a w2k3 machine either during startup (before the mouse cursor or CTRL-ALT-DEL login has appeared), or if I'm lucky enough the machine boots, but sooner or later the same BSOD appears.

The message is :

IRQL_NOT_LESS_OR_EQUAL

I haven't been able to get the Stop Code as the screen disappears too quickly and it reboots (it never dumps the core). Pause isn't holding the blue screen.

Event Viewer is showing no unusual activity immediately prior to the crash. The services being started and activities taking place etc. are wildly different between crashes.

The Bootlog indicated that one crash occurred after bluetooth drivers were loaded. I didn't need those, so removed them, but the problem persists.

I suspected memory. There are 2 x 512MB simms in 2 slots. I removed each one from either slot, and booted with only half the memory. I also swapped the slots for both sims, and also tried one at a time. In all cases the BSOD continued to occur (mostly at boot). I feel this rules out bad memory since I find it highly unlikely 2 memory modules and/or 2 slots would go bad at the same time.

I did however run memtest, and it reported bad memory — could it be the memory controller module on the mainboard?

No new drivers or applications have been added to the system prior to this problem starting. The machine has been running for 5 years without much incident.

I have done a complete system cleanup, scandisk (full), reg-check, checked CMOS settings, and removed a lot of old apps and junk in the hope of tuning it all up. I've also removed CD-ROM drive (not used much), reinserted the hard-drive in it's IDE slot, unplugged and plugged everything back in several times and physically cleaned it's innards. Checked fans are all working.

Problem is persisting!