...the first time I ran [the Windows Update troubleshooter], it showed an Error 1 accessing Windows\SoftwareDistribution, access denied.
Is it the same computer (not necessarily the same problem) as in this previous thread of yours?...
I am using Norton AntiVirus 18.5 (i.e. 2011) and its firewall...
http://answers.microsoft.com/thread/2b0be1bd-0274-e011-8dfc-68b599b31bf5
PS: Norton AntiVirus does not include a firewall - but Norton Internet Security & Norton 360 do. See http://community.norton.com/t5/Norton-Internet-Security-Norton/Does-Norton-AntiVirus-have-a-firewall/td-p/995345