If One Care is asking you to subscribe go ahead and select "Subscribe" and when asked for a Live ID enter the Live ID and password associated with your One Care subscription and One Care will activate. If it does not recognize your Live ID as having an active subscription asociated with it you are using an incorrect Live ID. If you cannot recall the Live ID associated with your subscription you can contact support for help with retrieving the correct Live ID. 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