I have Avast Free 17.5.2302 (build 17.5.3559.0) on Windows 7 Home Premium SP1.
After booting the Behaviour Shield turns itself off and must be manually restarted.
Looking in this forum I found two solutions: Repairing and/or Clean Install.
Both solutions doesn’t work.
So everytime I must check . 
Is there an other solution for solving this problem?
Does it happen at every boot or, only occasionally when you boot ???
The occasional problem is know and will be fixed when Avast 17.6.xxxx is released.
Hi, thanks for your reaction.
It’s indeed occasionally, but today I boot the PC 6 times and 4 times the shield turns itself off.
I’ll wait for the next update (and hopefully with a solution).
I can only pass along what I was told:
“hi, we made some improvements in service starting (based on your etl log file), so the problem should be gone in 17.6”
I’m as anxious to see a fix as you are. 
Thanx. Have a good and peaceful day.