Forget Windows completely, does it boot to a MS-DOS prompt? I highly doubt
a Windows driver is allowing or disallowing a specific CPU speed to work.
It could be more like a BIOS update to the motherboard that may have caused
some problems. Do you overclock? (which I don't by the way), maybe you have
the FSB too fast and with the multiplier on the 2000+ CPU, it's causing a
problem.

Just some ideas, but in any case, I'd try to boot to a DOS prompt first.

~ Adrian ~