serial number will be used to download the product from https://fileconnect.symantec.com if u have already downloaded it , u would have recived the serial when u brought the product. for activating license follow this document, it has all the info. Activating your Symantec Endpoint Protection 12.1 product license http://www.symantec.com/business/support/index?page=content&id=TECH162623