I have a similar problem with Avast 5 to those on several threads, like
http://forum.avast.com/index.php?topic=55894.msg473057#msg473057or
http://forum.avast.com/index.php?topic=54761.msg463369#msg463369. Only the sites listed on Expert Settings –> Exclusions can be accessed. I just couldn’t find any file at c:\Program Files\Alwil Software\Avast5\Setup\avast.setup, as mentioned on one of the threads. This Setup folder is empty with only one empty folder (INF).
I tried what I read on the threads, but no chance. It is the same with FF 3.6, IE8 or Chrome. I think the problem is related to some malfunction between Avast 5 and the firewall because Filezilla and Cobian Backup work fine, but I don’t what or where. Outlook 2007 has a similar issue; it sends mail maybe because I set the app not to scan outward mail, but it does not receive without turning the Mail Shield Off.
I deleted the firewall exceptions, but it didn’t ask again.
I use the built in firewall on Windows 7 only.
I also have Malwarebytes and Windows Defender.
This happened only after updating to 5.0.661.
Edit:I solved the problem. I post it here because in my case it was simple and may help others. I uninstalled Avast using aswclear5.exe, and before reinstalling I cleaned all registry keys and strings referring to Avast. After it was OK, and I just replaced the settings with my own, saved before the issue.