Mattlock2, it's very simple, just follow these steps:
1. The moment your system starts booting up, press or hold the Delete botton until you see screen come up (usually blue) that gives you lots of options. If that doesn't work, try F2, some computers have different commands to enter the BIOS.
2. From there, try to find the options for "First Boot Device, Second Boot Device, etc.". Once you've scrolled down and highlighted the "First Boot Device" option, press enter and you should have options to boot from certain devices (like your Hard Drive, CDROM, etc.).
3. Set your "First Boot Device" to "CD-ROM". Stick your WinXP CD in the CDROM.
4. Exit the BIOS saving your changes.
5. Your system will reboot, and you should be asked to "Press any bottom to boot from CD..." Once you do that, you're set to install WinXP.
However, since you want to format your Hard Drive, you should see an option for deleting a partition on your hard drive. Once you find that option, choose to delete the partition on your hard drive. It'll ask you a couple more times if you really want to delete your partition, and you want to say yes. Once that's done, you can press enter and you should be fresh installing WinXP from there. Btw, choose "NTFS (Quick)".