Run Windows Update and install all updates marked critical or important. Then use the link on this page to "Reinstall Windows Live One Care on your computer" -
http://onecare.live.com/standard/en-us/install/addcomputerinstall.htm To activate, open One Care, select "Subscribe" and enter the Live ID and password associated with your subscription when asked for a Live ID. If you receive an error message during installation please post the exact error message or 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