I need to install my old Norton Ghost 15

I bought Norton Ghost 14 at 2008 and I made an UPGRADE to Ghost 15 at 2010. I have my Product Serial Number, but when I try to activate my Ghost 15 product, I received a message about my Serial Number is wrong. What can I do?  I really need your help because I need to backup my PC and I don't want to buy another product if I already expend in Ghost.

TIA,