SD, do you have Spybots immunizer actioned?
That's maybe another area worth looking at.
I don't use S&D, so am in no position to say, but the reports exist, even if they are inconclusive. I wonder if S&D could somehow be blocking a range of sites that could be required to update.
I do use SpywareBlaster, all protection enabled, and have no problems.
That firewall might be deserving of more attention, even though you've tried disabling it. What has helped with ZA (another firewall) in the past with most users has been to remove any Avast entries, and cause them to be re-created as Avast asks for access. Is it a rules or applications-based firewall? Is the Windows firewall off?