You do not heed a product key to install One Care on additional computers. Your One Care subscription is linked to the Live ID and password used to set up your One Care account. To add additional computers install the trial -
http://onecare.live.com/standard/en-us/install/install.htm After installation open One Care, select "Subscribe" and when asked for a Live ID enter the Live ID and password used to set up your One Care account and it will activate.
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh - Microsoft Security Essentials