Boot into the BIOS, and set the CD/DVD drive as the first boot device.
Insert your new XP disc.
Restart the PC, and look for a message to 'press any key'. Do so at that time.
It will start the format/install process.
Make sure to delete any existing partitions and do a full format, if you want a clean installation. Follow the rest of the onscreen directions, and you should be all set. If you have any problems, just let us know.