Also having the same problem. Started occurring after recent program update. Also, I used a product key to upgrade from the free to the Security Pro version. The miniport shows up in the device manager in one of two ways. For a while, it displayed the full name (avast! firewall NDIS Filter Miniport) under network devices, and every time I tried updating the drivers, it would say the drivers were found but access was denied. Now, the device just appears as unknown device and reads "The drivers for this device are not installed. (Code 28) Access is denied." in the properties. I will run a hijack scan and post the log here soon.
Update: I tried uninstalling the device, and avast! seems to work fine. Is this device really necessary for the firewall? Avast still shows the firewall as operational and my internet connection has not disappeared like some people have found.