Check that your computer date and time are set correctly. If the date and time are correct go to billing
http://billing.microsoft.com sign in using the Live ID and password associated with your One Care subscription and verify your subscription end date. If your subscription end date is correct in your billing account but shows expired in One Care please contact support. 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