You do not flash a BIOS on XP, or any other operating
system for that matter.
A BIOS is more fundamental than the operating system, so
it must be changed before the operating system is active.
To stress this point, you could flash a BIOS without any
hard drives attached (hence no XP, no 98, no Linux, etc).
If an OEM PC (e.g., Dell, Gateway, Compaq, etc) check
their support eb site for details, or send an email for
support. If a home-built PC, or if a custom shop PC, go
to the website of the motherboard manufacturer. Also,
reading the motherboard manual is often useful.