Right firstly we need to make sure we have backed up everything which is required as once you start the XP reinstallation process it will delete everything from your computer.
Ok lets do it together.
1: Insert the OS Disc which has been sent by DELL into the DVD Drive
2: Restart your computer
3: It will come up with an option Press any key to run from CD/DVD
4: Press any key, I usually press the arrow key on keyboard
5: Now the XP Installation process will start
6: You will see a blue screen with some bits working on the status bar, let it finish it.
7: Once the reading bits finishes, the blue menu comes which says press Enter to start XP or Press R for repair
8: Press Enter
9: Now you'll see another screen with Windows XP partition, asking you to press D to delete the partition. Press D and then it will ask you to press L to delete it fully. Press L
10: Now it will come back to the partition page. Press enter key
11: Once you press enter, it will ask you to either use quick NTFS or full NTFS.
12: Press quick NTFS and let it install XP
13: After a while it will come up into a graphical interface, upon menu choose the language (if you are in America then you don't need to change anything)
14: Now it will ask you to enter 25 digit key. This key you can find under the laptop or side of your desktop (it is a sticker from Microsoft!).
15: Once entered you are done.
It will install the XP.
If you need any further assistance please do let us know and we'll do our best to help you
. Perhaps I can come into the horse forum chat if you have two computers and will help you install it while telling you live.