Since you re-installed your OS, I'm guessing that you also uninstalled Avast and re-installed this as well. Did you use the Avast Uninstall Utility Tool to uninstall prior to uninstalling your OS?
Have you changed any of the default settings of Avast? I would not disable any other other shields at this point. We need to find out what is causing your problem.
What other security software do you have on your machine currently (firewall, other software)?
Are all your Avast Shields up and running? Are you using Avast v.5.0.677? What product are you using (Free, Pro, AIS)?
Have you tried an Avast Repair:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
Do you have any unnecessary items running at Start Up that you can disable (not delete).?