Port 80 blocked

I recently went to a customer who claimed that he couldn't view web pages and yet email and messaging worked as usual. I thought that it just meant that I had to reset the web browser's security (Internet Explorer) and check for proxy servers, etc. However it still didn't work. Other computers on the router worked well, so it wasn't that. I spent two hours checking every thing such as reset Winsock settings (netsh winsock reset), checking the hosts file, etc.

I searched the web for clues but found none, though I did download and ran a couple of network repair utilities such as LSPfix. Still no joy. I then performed a complete virus and spyware scan. That revealed evidence and removed Trojan.MeSPAM but still didn't fix web browsing. It was looking like a Windows reinstall was required which seemed over-the-top for such a small, but vital problem.

It was definitely TCP/IP HTTP port 80 that was being blocked. DNS servers were OK and resolving. Other nework access was fine. The AVG AntiVirus was not updating, but I believe that was also based on port 80. Booting into Safe mode and web browsing was fine. I searched for dormant firewalls and other strange services but found none.

As a final desperate act, I removed all the software applications that had anything to do with web browsing or the network such as; Flash, Shockwave, MSN Messenger, Yahoo messenger, ICQ, MSN Family Safety and all the other MSN applets, AVG AntiVirus, etc. Reboot and web browsing is fine.

Reloaded all the stuff apart from MSN Family Safety which the customer didn't require and surfing the web continued to be fine. Update to Internet Explorer 8. Still OK. So what was the problem?