You have found one of the reasons a clean, fresh install is better than an upgrade. If you wipe the disk, there will be no system left to upgrade from.
First of all, are you sure your computer meets the requirements for XP and there are available drivers for all the computer's components and the peripherals you are using? (printer, sound card, video card, etc)
Check and see if your system can be upgraded to XP here:http://www.microsoft.com/windowsxp/home/up...ng/default.mspx