There is no upgrade path from the 32-bit version of Windows XP Pro
to the 64-bit version of Windows XP Pro. Assuming you have the
hardware requirements to run a 64-bit version of Windows XP Pro,
you would need to purchase a 64-bit version of Windows XP Pro
and perform a "clean install".
Purchase WinXPx64, and then simply boot from the WinXPx64 installation
CD. You'll be offered the opportunity to delete, create, and format
partitions as part of the installation process. (There is no upgrade
possible from a 32-bit OS to a 64-bit OS.) You may need to re-arrange
the order of boot devices in the PC's BIOS to boot from the CD.