Norton Product key is not valid for my third computer!?? Please help

Hey there good people!

 

Today my brother asked my to install Norton 2011 on his computer but sadly the product key is already being used? I know for sure that my product key can be used for three computers " says on my account" ? I use mine , it is good, my mother uses her's it is also good, how come the third computer can be activate Norton.

 

Computer is Windows 7 Ulimate 32bit

 

Thank you good souls