The most likely cause of this is other software conflicting with One Care. If you have other antivirus or antispyware software installed on your computer, uninstalling it should resolve the issue. If you have no conflicting software on your computer or if removing it does not resolve the problem 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