Xp coa changer from Microsoft home page?

I had to do a system restore and used my mates CD for XP. Unfortionalty i entered the wrong COA. Even though it was accepted and continued to load, now i have come to the updates it has declined due to the code. My COA on my pc i need to enter now but i can not find the Microsoft screen where i can re enter my original COA. I have seen it before, but can not find it anywhere on the microsoft website. Please help