Try the OneCare cleanup tool first -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2
You will need to save the tool to your desktop and then right click it and select "run as administrator." Once it completes, reboot and try to install OneCare again.
Note that when OneCare is installed, Windows Defender and the Windows Firewall are disabled as this functionality is taken over by OneCare.
-steve