There are hardly any settings in your BIOS which will render your system useless, apart from of course those which determine your CPU frequency such as the multiplier, FSB and voltage settings- if not configured safely or correctly. Not all BIOSes allow configuration of these settings however. You'll need to check.
Formatting your PC will do nothing but cause you additional hassle. Formatting your PC will not solve this issue because it's not your Operating System which is the issue, it is your BIOS settings.
Sorry, I thought for a moment you have a Pentium 4 Processor
. AMD Athlon XP Processors do not have thermal protection integrated onto the CPU. So it doesn't throttle like a Pentium 4 processor or shut the system down like an AMD 64 processor. If temperates go above the critical 80°C mark then the AMD processor would very simply cook to black smoke. But your processor is below the critical mark, and temperature is completely out of the question.
Basically, after your BIOS upgrade, the BIOS hasn't set your FSB to 166Mhz (effectively 333Mhz). It has set it to 99Mhz.
The bus speeds need to be corrected to the original parameter, which in your case is 166Mhz. All AMD Athlon XP Processors (excluding some newer barton cores) are configured at 166Mhz bus speed (effectively 333Mhz). You need to go into your BIOS and correct the bus speed from 99Mhz to 166Mhz. This won't damage your PC in anyway for it is not overclocking, it is correcting your system to it's original configuration. However, you may not be able to configure bus speeds in your BIOS.
Do this after you've tried resetting your BIOS by removing the BIOS battery from the motherboard, and placing it back after a minute or so. Resetting your BIOS won't damage your PC either.