If One Care installation fails see this post to troubleshoot installation issues -
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/ba26b781-da18-4ce4-be8c-22647bd5d8fc If you are seeing an error message please post back the error number and text.If One Care installs but asks you to subscribe it just needs to be activated. To activate open One Care, select "Subscribe" and enter the Live ID and password associated with your One Care subscription when asked for a Live ID.
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh - Microsoft Security Essentials