OneCare cannot be installed on Windows 7, so it cannot be added to your Circle.
See this post for details on OneCare end of life as well as a link to the free Microsoft Security Essentials for virus and spyware protection on your Windows 7 computer.
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/0000c782-ed1b-4ed3-8afb-87989bb9638c-steve
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