You don't need the product key. To activate a second or third computer you only need to use the Live ID and password originally used to activate your subscription. To install One Care on another computer download the One Care trial here -
http://onecare.live.com/standard/en-us/install/install.htm To activate, open One Care, select "Subscribe" and when asked for a Live ID enter the Live ID and password originally used to set up your One Care account and One care will activate.
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh