You will have to call them to activate the windows. You will have to call them every time to activate it (as the number generated varies every install). Microsoft will ask you how many PC's its on. Say one obviously. They aren't stupid though. The copy you have has been installed before. (the auto activate feature works 3 times before requiring phone activation). If someone else registered the copy you are trying to activate you will have a problem. Microsoft isn't stupid, if I can read between the lines so can they...