I had this problem as well, it started about the same time that the original post was posted.
I have been using Avast Home for years, and recently built this computer maybe a year ago. I was using Comodo firewall, Ad-Aware Anniversary free edition, and PCTools ThreatFire. First I thought that the reason I was unable to use my connection was dew to my USB wireless going out. So I replaced it with a D-Link PCI card, and that did nothing to fix my problem. So I did a reformat of my computer (I was having other problems too). After a clean reinstall of Windows Vista x64 I proceeded to install the latest versions of all my drivers, security programs, and Firefox. The problem happened again. I had no viruses and no malware, as this was a clean system.
I thought that a windows update or a driver update might be the culprit. So I reformatted again turning off the Automatic Update feature built into Windows. Installed all the old drivers and tried the internet, and it worked fine (only trusted sites). I then reinstalled all the security software (Comodo, Avast!, Ad-Aware, ThreatFire) and FireFox, and after about 30 minutes of surfing the internet stopped loading in both FireFox and Internet Explorer. At this time I still had not done any Windows updates or driver updates.
Knowing that a Windows update or a driver update was not to blame, I reformatted again. This time I did all the Windows updates and all the driver updates and the internet worked fine. So I tried reinstalling each of the Security products one by one. I started with Comodo, and the internet worked fine. Then Ad-aware, and the internet worked fine. Then Avast!, and the internet stopped responding. So I uninstalled Avast! and the internet was back. Then I tried ThreatFire and still no problems.
I thought it might be a conflict with Avast! and another security product. So I did another reformat, and this time I updated Windows and all the drivers. I then installed Avast! by itself and i lost my internet again.
I uninstalled Avast! and installed ZoneAlarm, WinPatrol, and Avira. Everything is working fine now, but I HATE Avira. Please fix this problem so I can switch back.
Specs:
MSI K9A2 Platinum
AMD Athlon 64 X2 5600+
8GB RAM
NVidia 9600GT 512MB
2 DVD optical drives
2 250GB HDs RAID 0
650 Watt PS
D-Link wireless NIC
320 GB external USB HD