I have a Norton Internet Security currently installed on my PC and according to my norton account i should be able to also use this subscription on two other computers. However, when i attempted to use my product key on my sister's computer, i was told that my product key was invalid, repeatedly. I even went to my norton account and copied and pasted the key on there into the space where the product key was supposed to go. I am completely stumped on what to do. Any help would be great.