Boot into Setup (Bios) change the boot order to be CD rom drive first boot device. The XP disc you are using, is this a burned disc, a restore disc, or is it a Microsoft Windows XP CD? Put the disc in the drive and restart your computer. Like Pip22 said, if you don't see the message "Press any key to boot from CD". it means your PC is not booting from the CD. Either the CD drive is not the first boot device, the CD isn't bootable, or the CD drive is not reading that disc.
Try booting that disc in another computer.