When you first turn on your PC you will see a message "Press F2 for Setup" or similar (the exact wording and the key to be pressed will vary according to the make and version of the BIOS). Press whichever key it say, and explore the menus. In one of them you will be able to set which devices are checked for bootable images, and the order in which the are checked. You need to set it so that the CD Drive is the first to be checked. With some BIOSs you also need to enable the option "Boot from other devices". Once you have done that, save the options. Put the XP CD in the drive, shut down the machine completely (leaving the CD in the drive). When you next turn the machine, it will either boot straight from the CD, or it will show a message "Press any key to boot from CD" (or words to that effect).