There are two differnent things with windows XP. first is registration, which you do not have to do if it asks, you can just cancle. The second is Activiation, which you DO have to do, else your OS is disabled in 13 days or so. The code that you have with windows is known by microsoft. This code is associated with the specific hardware in your machine. It knows you have X motherboard and y network card, etc. etc. If the hardware assiciated with the machine changes too much, you have to reactiviate. AFAIK you cannot have two seperate machines activiated witht he same code at the same time, though you certainly can move your license to another machine, which will require you to reactivate, and probably, deactivate yolur old one.