The best thing to do in this situation is ring Microsoft windows support and explain your problem. If you registered the previous copy then it shouldn't be too much of a problem attaining a new key.
Google "Windows XP Activation Tool" and run it in safe mode on the administrator's account. Then restart your computer. It will fix any XP based activation problem. If you are running Vista or higher, then you may be out of luck, if Microsoft chooses to not give assistance.