A couple of things you can do before you reinstall:
1. Download and run Keyfinder (
http://www.magicaljellybean.com/keyfinder/)
This is display the product key of most Microsoft products that are installed on the computer (included Windows XP)
If the key matches the sticker on the bottom, then good.
If not, write down the product key somewhere so you can try this key if the sticker key doesn't work.
2. Find out which version of Windows you are running. XP Home or XP Pro
You can do this by clicking on START then right-clicking on My Computer then clicking on properties.
(or Start > Run > sysdm.cpl)
It should say "Microsoft Windows XP Version 2002" etc etc
3. Download the OEM version of Windows version from step two.
As far as I am concerned, this is a legit use of bittorrent because it is downloading something you already own.
Search your favourite torrent site for: Windows XP Professional OEM (or Home OEM)
This is pretty neat because they'll usually include Service Pack 3 with their CDs
4. Burn the CD using your favourite iso burning software. I use imgburn (
http://www.imgburn.com/)
5. Boot from the CD and try to install Windows using the product key that you have.
If you want, you can always make a backup of your current install using clonezilla and a USB harddrive. That way, if the CD install doesn't work but you formatted your drive, you can always go back.
EDIT: I should note that you'll probably have to download the drivers for all of your stuff to get things working, again. With pre-installs, sometimes it is just easier to uninstall all the junk using the add/remote programs.