1) Determine if you can boot from CD by placing the Windows XP CD (or other known bootable CD) in your CD drive and reboot. If the system starts from the CD, great, if it just goes back to your usual desktop, not so good, you'll need to dig further into how your system works.
2) Assuming you *can* boot from the CD and have the Windows XP CD in during boot, keep an eye out for the message "Press any key to boot from CD". Earlier XP boot CDs did not have this, if I recall correctly. If you *do* see it, press a key. This will start the setup process.
3) Read each screen that comes along, and answer accordingly. There's really not much to do but accept the license.
4) You will eventually be shown a list of available partitions on your hard drive. You want to DELETE all of them. Follow the instructions. It's something like D to delete, then L to confirm.
5) Now CREATE a partition, accepting all defaults
6) Format NTFS (NOT Quick Format NTFS). Depending on the size of your drive, this may take a while.
7) Continue reading screens and following instructions until your installation is complete.
8) Get yourself anti virus software, anit spyware software, and a firewall. See below for suggestions and links for HOME use.
AVG Anti Virus: http://free.grisoft.com
ZoneAlarm firewall: http://www.zonelabs.com (link near bottom to Free Downloads)
Spybot anti-spyware: http://www.safer-networking.org/