For all those having problems with NDIS drivers and or are unable to install firewall driver during installation I have found a solution. This solution works on windows 7 & 8.
On Windows 7
1. Boot into safe mode with networking by repeatedly tapping F8 just after turning on your system and select "Safe mode with networking"
2. Once windows is loaded start device manager by pressing start and type "devmgmt.msc" and press enter or go to Control Panel > System > Device Manager.
3. Open the properties for the NDIS driver/s that are causing a problem click update driver and select update automatically and windows should install the correct driver without getting the access denied error, as avast is not running. Repeat the process for any other NDIS drivers or unknown devices that are actually the NDIS driver that have failed to install.
4. Reboot into normal mode and hooray avast should now function correctly with all features and no annoying missing drivers.
On Windows 8
1. Booting into safe mode is not as easy on windows 8, In normal operation from your start screen type "msconfig.exe" and press enter. Select the boot tab from the top and tick safe boot and select network from underneath, press ok and reboot. If you can't boot into windows its a little trickier Windows 8 monitors the startup procedure and when problems are detected it automatically takes you to the new recovery mode. At this point you will be presented with the message: ‘Recovery. It looks like Windows didn't load correctly'. Select the advanced repair options and choose Troubleshoot > Advanced options > ‘Windows Startup Settings' then Restart. Your computer will boot into an ‘Advanced Boot Options' screen in which Safe mode with networking can be selected.
2. Once windows is loaded start device manager by pressing start and type "devmgmt.msc" and press enter or go to Control Panel > System > Device Manager.
3. Open the properties for the NDIS driver/s that are causing a problem click update driver and select update automatically and windows should install the correct driver without access denied error, as avast is not running. Repeat the process for any other NDIS drivers or unknown devices that are actually the NDIS driver that have failed to install.
4. Reboot into normal mode and hooray avast should now function correctly with all features and no annoying missing drivers.
If this problem occurred while avast was OK and you have uninstalled, used awsclear and reinstalled and are getting the error "Unable to install firewall driver" Simply use system restore to restore your system to a point before you uninstalled and then follow the instructions above
Hopefully this will help anyone else who is at their wits end with this problem as I have been trying to solve this problem.