Same problem with me.
I´ve installed Windows 8 from scratch, and also Avast! Internet Security. After reboot, Avast! tried to install NDIS Firewall about 20 times. My network doesn´t work anymore. After full uninstall of Avast!, Internet comes back, but the system are now taking more time to start.
But are you all kidding me? I think I don´t have do to technical stuffs in my system to make Avast! work. Maybe with free antivirus versions, but not after paying a 2 years license.
After contacting technical support from Avast! I received the procedures to do a "succesfully" installation of Avast! in Windows 8:
* Uninstall all other antivirus software already installed onto your system; (in my case none was installed)
* Deactivate any firewall running onto your system; (Windows Firewall deactivated)
* Download and install the latest Avast! Internet Security; (downloaded and installed)
And after that, no network again.
I think Avast! just doesn´t support Windows 8 already, but it is a shame to say that.