Set your bios to boot from the CD drive and make sure your XP CD is in the CD drive. You will be prompted to press any key to boot from CD, press the 'any key'. Watch the lower part of the screen (it is a status bar) and at the prompt press the required key for Automated System Recovery and follow the prompts. This program will scan your system and source the required files from the CD. It will then advise a restart and automatically reconfigure the system files on the reboot.
If you miss the ASR prompt it will automatically start loading the setup files for installing XP.