Had a fun time this afternoon diagnosing a problem on my father's WinXP laptop. He could send and recieve e-mails, and connect to secure (https://) websites, but not plain websites (http://). It looked like something was blocking port 80, but Windows firewall wasn't and there didn't seem to be any other firewall software installed.
He is using AVG AntiVirus, paid for, but not with the firewall component.
Eventually tracked it down to a process called avgnsx.exe which seems to be the thing that both LinkScanner and Web Shield use in their work. Turning off both these protections and suddenly we could browse the web again!
AVG have some comments, and a possible fix, on their FAQ pages:
Interestingly, and perhaps related, we also saw repeated requests to reboot following AVG updates. This might be related to the above problem, and is also dealt with in the AVG FAQ:
For those who are interested, I found out what was blocking port 80 by installing the excellent, and free, TcpView from Microsoft Windows Sysinternals:
Run this with "Always on Top" selected, and see which processes do things when you try to navigate to a web page.
Search things: https not http, port 80 blocked, AVG, no firewall, many web pages not found.