what you do is
1. Save all your data you want to keep on a seperate hard drive or whatever means you have to save. Make sure you know the size of the drive you want to format and also that you have all the drivers for your computer. I.E. motherboard drivers, accessory drivers, ect. Also make sure you have the cd-key for windows xp.
2. Go in the bio's and change the boot priority to boot from the cd rom first.
3. Insert the disk and it will say boot from cd-rom, hit any key to continue so just hit a key. It will now load the setup files and everything needed to install xp.
4. Next it will say hit enter to continue and then press f8. It will now ask you what you want to do. What you want to do is continue to install windows on the drive that already has windows on it. Just keep going through those screens saying to install it on that drive anyways untill you get to the screen that asks if you want to format the drive or leave it as is. Format it choosing ntfs quick format.
It will now format and load all the rest of the setup files needed to install. All you will have to do now is put in your name, computer name, time zone, ect.
If you have never done this before get somebody who is local for you that can help guide you through it if you run into problems. Its really self explanitory but it can get tricky depending on what equipment you have in your machine.
Intel Q9550 ( 3.84ghz OC on air)
Asus Rampage Formula
OCZ Platinum dual channel memory 8gig
Vista Ultimate 64bit
Radeon 4870 512mb