Backup all your stuff, photos etc.
Then make sure you have your Windows XP Disc.
...and your windows "Product Key"
Pop in your XP CD and restart your computer now you will have to make the computer BOOT from that, otherwise you will just go striaght into windows.
When the computer is starting keep tapping the DELETE key and this will bring you into the BIOS.
Change the boot device from Hard drive to CD and make sure you save this change. Consult the Motherboard manual for instruction on how to do this or have a look at the motherborads website.
You will now BOOT from the CD and you can start formatting. You will have to delete the XP partition and then create a new one or two or however many you want. Then just install windows from there...
The XP setup from the CD is easy to understand.
Heres some info and screenshots click here