Make sure you have done all of your backups and that ypou have your Windows product key before commencing.
Pop your XP CD in the CD drive and restart your computer.
Now, you have to set your computer to look at the CD Drive when it is starting so when the computer is starting keep tapping the DELETE key.
You'll now be in the BIOS
Now you have to change the way in which your computer BOOTS, in this case you have to make it look it at CD DRIVE. So see if you can find a BOOT SEQUENCE. Change this boot sequence to only look at the CD Drive.
Have a good look.
Now exit the BIOS and make sure you save the changes.
Restart and your computer will now look at the CD Drive and read the XP CD. When instructed press an key to continue.
Now give it a few mins.
Now...Just follow the on screen instructions.
Its best to delete the XP installation of the drive instead of XP just rewriting it.
So press to "D" to get XP of the system...
Then when your ready just press ENTER to install XP.