You don't flash BIOSes in XP. You must first obtain or create a bootable floppy. Go here for openers.
boot
Then you must carefully read how your donloaded file are put on to that bootable diskette and "run." The BIOS flash is not hard, but if you don't do it right, you can seriously cripple your machine.
Most BIOS routines allow you to copy and name your existing BIOS. Do that.
Basically the routine is to run the floppy, and execute the flashing exececutive file.
Be sure and read all warnings and readme's.