Set the BIOS to boot from CD-ROM, insert the XP CD, and
reboot.
Press a key to boot from CD when prompted, and XP setup
will start, allowing you to format in the process.
Simply boot from the WinXP installation CD. You'll be offered
the opportunity to delete, create, and format partitions as part of
the installation process. (You may need to re-arrange the order of
boot devices in the PC's BIOS to boot from the CD.)