You'll need to go into the PC's BIOS. This varies from one computer to another, but usually it's either the DEL, F1 or F2 key. You may need to repetitively press the key once the PC is powered on. On some PCs, the BIOS displays the key to press when you power on the PC, e.g. "Press DEL for BIOS setup", etc. just before booting. Alternatively, try pressing the F10 key to see if it gives the option to choose what to boot. If you get a boot menu, chooes 'CD'.
In the BIOS, have a look for a section with boot priority and change it such that the CD drive is the first option. On some BIOS's this in in a section called "Advanced BIOS settings", "BIOS features setup", etc. The best thing is to check through each menu until you see one giving boot options. Once you change the boot priority and have the CD loaded, you should get a message like "Press any key to boot the CD . . ." When you see this message, press a key to start booting the disc.
In the Windows setup, choose "Install windows" and then on the next screen, choose the repair option. The repair process looks much like installing the OS from scratch, but once it completes you will still have everything installed as before. Be sure to have your Windows XP key handy, which should be on a sticker towards the rear of the PC.