This error means that the antivirus component failed to install due to a corrupt Defender install on your PC. See this article for troubleshooting help:
http://support.microsoft.com/kb/930162
If the steps contained there do not help, please contact support:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve