Other people have accessed my account

Hello,

I recently logged onto my PC and there was a message displayed stating that my Norton 360 trial had expired.  This was strange as I have had 360 installed on my machine for over 4 years.  When I logged onto my Norton account online I discovered that five other computers / users completely unrelated to me had used my product key and were accessing 360 and using the available seats.

I was really concerned by this security breach.  I contacted Norton and was told but the customer representative that either a) the shop where I purchased the software had shared the product key - v unlikely given that I bought it from PC world in sealed packet over four years ago  b) I had shared my laptop and Norton product key details - impossible.  I would not share my laptop as it contains sensitive data relating to my company and family. 

Norton have provided me with a new product key which solves part of the issue but does not provide a satisfactory explanation as to what has occurred.  I am really concerned that the software that I installed to safeguard my computer has been compromised.  

Please can you help and provide a clear explanation as to why other users, unconnected to me where able to access my product key and use this.

Thank you in advance.

Willip4