With XP you format from the CD itself. Set your BIOS to boot from the CD, insert the disk and follow the instructions for a normal install.
XP will load some files to your drive then as it inspects the drive it will inform you of the current install and ask if you want to repair [If your hardrive is partitioned, do you want to add to a different partition]. or format. Choose format then format the whole of the drive or into separate partitions as you wish. j.