You will not need to update your BIOS again after re-installing windows XP. Your BIOS is flashed onto a chip on the motherboard itself. It will remain, regardless of what you do to your OS.
It should be saved on the CMOS chip. If you get any obscure boot errors after flashing BIOS (CPU Change error, keyboard error, etc.), you should clear the CMOS, via the jumper on the motherboard, back to default and reconfigure your BIOS settings to match what they were prior to the flash.