It's really simple.
Once you've made your backups..
Put the Windows XP CD in your CD-ROM, turn the computer on. It's going to prompt you to press any key to boot to CD.
Once this happens, just follow the steps. Basically you're going to take the defaults and shouldn't need to do anything except pick a computer name and set the administrator password.
It's very very very simple. It takes about 30 minutes, of which maybe you need to sit there for 5 minutes answering questions and confirming it's what you want to do.
If you really need more details steps, you can do a google search on "how to install windows xp" and I'm sure you'll find screen shots and the like.