The product key is used for the original activation of your subscription and thereafter linked to your Live ID so you cannot re-enter the product key. To install One Care download the trial -
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 associated with your subscription and One Care will activate.
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh