enter BIOS ( Setup ) and set the first boot device as CD/ROM. Put your XP Disc in the drive and disconnect all external peripherals except the monitor, keyboard and mouse, and be sure those are not USB, or wireless. Any internal USB card should also be removed.
Once you set your BIOS to boot first from cd . . when it restarts you will see a black screen with a prompt "Press any key to boot from CD" . . do that and you will be able to delete any or all partitions and recreate one or more, then proceed to formating and installing XP.
That message can pass quickly, so have a finger on the keyboard when you boot. This will delete all data on the drive so be sure you have your important data backed up. The prompt will appear after every reboot, but do not press any key on subsequent reboots. The setup process will continue with no action required from you.