Are you referring to an update for OneCare? If so, see this post:
Information to help troubleshoot and resolve update problems -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1286736&SiteID=2
If you need help, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve