Windows Update should not be blocked by the OneCare firewall, so something else is apparently broken with the firewall following the update.
You should contact support for help in resolving this or simply uninstall and reinstall OneCare to see if the problem can be resolved with the "hit it with a hammer" solution.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve