I had the same issue when I first installed it, 5 TIMES with a freeze and having to shutdown via the power button. It happened when I used the Help settings and browsed the different sections of the interface.
I have the free version, which I installed over 4.8 for the reason of keeping my license key intact. That part worked.
I also ran the Check Disk system utility to clean up and fix any errors potentially from the freeze. Fortunately, none were found.
The workaround (for now) is upon reboot, DON'T OPEN THE AVAST INTERFACE FOR ABOUT 1-2 MINUTES. Let Avast "Call Home", check for updates, do its thing, which takes that amount of time. After letting the new version do that, I was able to browse without system hang.
There are too many people who are understandingly clicking on the interface too fast after rebooting, and I think this is part of the freezing problem. I've addressed that this issue needs to be the first bug fix that the programmers need to address.
Also, I have not had a need to reboot since doing this, but under the Basic Settings, UNCHECK the option that says something like "Use advanced interface." It even says that on some systems, with that checked, it may slow things done. It says changes will take place after restart.
Techs, what actually changes in the GUI if this option IS unchecked?
A HUGE plus is that I can "feel" its lighter side to the use of memory and less resources used than 4.8. There's one less icon in the system tray and less sub-menus compared to 4.8. Once that reboot issue is better resolved, this will be a great AV program!
Jack