Try directions above, but before trying to reinstall. Go into device manager and show hidden devices. Remove all the non plug and play devices, that are related to avast. Reboot, and then try to install avast, avast installs and does not prompt to reboot, Then start run msconfig in general tab select load system services in selective startup. Reboot again, wait for avast to load up. Then try start run msconfig again general tab select normal startup and reboot.
This is what worked for me. Hope it works for you too.