I just did a clean install of Windows 10 on my cousin’s laptop but after installing Avast, Avast service won’t run. I tried to reboot but it says it failed to boot. Same thing when I tried to restart again. It worked when I tried to do a system restore to before Avast was installed.
Try avast! repair then restart pc few times.Avast service will run.
I did the exact same thing (clean install of windows 10 and installing Avast FISRT without installing anything ) then reboot and windows isn’t able to boot now… wtf avast??
The almost same thing happened to me. I installed AVAST and it did a scan of the computer and wanted to do it during the startup of the system and after that is says that my drive is locked and i cannot start windows 10! And since its locked i cant install through dvd either… But on another hdd it went successfully but i want my old install back. What to do AVAST?
Guys I found a solution and worked for me ;D
Here it is…
Install Avast Antivirus but don’t restart the system
Now right clicking the avast icon on the task bar → avast shields control → disable until computer is restarted
Now right click on the task bar ->select task manager → select more details (if it is only in simple view) → select startup tab
right click on Avast! Antivisrus → disable it
Now restart your computer
Now manually run Avast
Now again select task manager → select more details (if it is only in simple view) → select startup tab
right click on Avast! Antivisrus → enable it.
So… now its done.
Check it now
Have fun 
If you’re running Windows 10, you need to use the latest version.
If you’ve just installed the latest version and one of the shields didn’t start, just reboot the system again.