BIOS update changed FSB speed

Which bios version were you running when you tried resetting the CMOS?

Turns out the answer is pretty simple, although not obvious. Despite what the sales literature said, the "real" FSB speed of the processor is not 266 MHz. It's 133 MHz. The following from Wikipedia sums it up:

"Athlon XP-M 2600+ 2000 MHz 512 KB 266 MT/s 15x 1.65 AXMA2600FKT4C "

"The Athlon XP uses a double data rate (DDR) front side bus meaning that the actual data transfer rate of the bus is twice its physical clock rate. The FSB's true data rate, 200, 266, 333 or 400 MT/s, is used in the tables and the corresponding physical clock rates are 100, 133, 166 or 200 MHz, respectively. The multipliers in the tables above apply to the physical clock rate, not the true data transfer rate. "

I figured this out when I realized that I had DDR 400 memory, and I could go up to 200 MHz, but no higher on the FSB setting.

So I just set the FSB to 133 and the multiplier to 15 and everything runs fine at 2GHz. Apparently I had done this before but forgot about it. Two lessons here:

1. Always write down your BIOS settings before an upgrade
2. Computer repair/upgrading activity should stop at midnight. "Let me try just one more thing" is not sufficient reason to continue to 3 AM.