It's likely a matter of pressing F8 repeatedly when you boot. If your BIOS doesn't have a 'Quick Boot' function (normally F8) then you need to instruct the BIOS which device to boot from. Hitting the "DELETE" key, or sometimes even the "F2" or "F12" key when you first turn the computer on should present you with the BIOS options. Once in the BIOS, you want to assign your DVD drive to be first in the 'boot priority' menu. Look in the advanced BIOS menu. If the XP CD is in the drive, you should be able to 'boot from CD' next time.