Here is my solution to your problem although you may not like it. I would use a program like Keyfinder to find out your registration key for your windows install. I would then find a ID program, or use system information, to find out what hardware you have in the computer. Now you will go buy a Windows XP OEM CD from Ebay. If you buy a OEM CD that still has a usable registration key and since you already have your own key, you can resell the disk after you get done with it. You reinstall XP from the CD but use your own registration key. Then you are back in business.
The caveat here is 2 things. One time i used the wrong CD (SP2 on a SP1 system) and Microsoft would not let me register the computer, so i had to use the key that came with the CD. I still can't figure out why microsoft did this other than they thought i was using someone elses CD and reg key. The second thing is you will probably have to find the drivers needed to complete the installation and that can take some time.
This takes me to where i am now. I have been using Acronis True Image to make restore CD for peoples computers( that do not have restore CD's). This works well for the computers because it also includes all the programs they have installed on the computer. I also repeatedly tell people to backup, backup and backup again anything they do not want to lose.