Put the Windows XP disk in the cdrom, and reboot the machine. If your BIOS is set to boot from cdrom, the windows startup will begin, and tell it to format the entire drive. Linux will be gone, and all that will be there is windows xp.
If you put the cd in, reboot, and linux starts again, then your bios needs to be changed to boot from cdrom. Bios is usually entered very early in startup by the delete key, or F1 F2 depending, just read the screen. Once in Bios, go to boot, and make sure cdrom is used before any of the hard drives. It may be under advanced CMOS, there are plenty of things it may be under, just get in BIOS and look for the boot order of devices.