I have been using an Intel board D850MV with XP Pro for 2 years. It started crashing (locked-up) after hours of use. Now it crashes at any point during start-up, such as SI splash screen, Windows operation, BIOS adjust attempt. It even crashed while a Norton Virus (boot-up) CD was scanning. When it crashes, it will not boot again for hours unless I unplug the main power harness to the board. I have tried different video cards, disconnecting the floppy, CD and DVD drives.

I tried using only 2 memory cards at a time (256Mb each), neither pair worked better.
Could the thermal diode be defective?
The chip is a Pentium 4, 2.2Ghz

If it shuts down within the first couple of minutes, regardless of what you are doing, then it could well be a power supply problem (which is perhaps why you have to unplug the mains before it will boot up again).