I had the same problem for three weeks. I sent emails to Symantec ( Firewall ) and Microsoft. Both were not able to provide an answer. Eventually, after asking Microsoft repeatedly if a repair of XP might help ( and getting no answer ), I did a repair of XP and that cured the problem. SFC failed to sort it. The cause of my problem appears to have been down to a corrupt file which SFC did not sort out.
Does not help if you are not on XP though !!