If you had other antivirus software previously installed, go to the vendor website and look for an uninstall tool. You can also try the One Care Antivirus repair tool -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1695761&SiteID=2
If you are still unable to turn on the antivirus protection please contact support. How to contact support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2