You do not need to re-enter your product key to activate One Care. To activate One Care enter the Live ID and password associated with your One Care subscription when prompted. If you cannot recall the correct Live ID and password you will have to contact support for help. How to reach support -
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh - Microsoft Security Essentials