How can I change the product key on my XP?

I have a XP, bought in Germany. I changed the power supply to 110 so I can use it in the USA where I live now, but I have the problem now that whenever I turn the computer on that it tells me that I might not have the original Microsoft ware installed. Is there a way to see why the product key changed and how do I fix that? Can I change the product key to the one that is on my computer?How can I change the product key on my XP?
The product key didn't change, the hardware fingerprint did (they have to match). Call Microsoft (they do that every day for people who change hardware).How can I change the product key on my XP?
http://articles.techrepublic.com.com/510

That is a fairly advanced article on steps telling you how to change the product key, if your not comfortable doing that there are third party tools that allow you to change the product key. Checkout Magic Jelly Bean, and as far as figuring out why the key changed, there is no easy way to go about that besides looking through logs and event viewer.How can I change the product key on my XP?
The quick guide to change your Windows product key:

http://www.top-password.com/change-produ