Go into the BIOS and set your first boot device as your CD drive, save and exit.
Pop your XP cd into the drive and let it boot. The cd will run the installation from there.
Words of warning.
This method will destroy ALL existing data on your drive, so now is the time to backup anything you need.
If your hard drive is SCSI (unlikely), SATA (possible) or is using a RAID controller, make sure you have the drivers available on a floppy disc and press the appropriate F key (F6 from memory) during installation. If this applies, you only have a few seconds right at the start of the installation to do this, so watch the screen carefully.
click here for some screen shots.