See this post for the KB article to troubleshoot install problems - http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/ba26b781-da18-4ce4-be8c-22647bd5d8fc/If none of the information from there helps, please contact support:
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90-steve