Behaviour Shield is frequently reported to have failed to start. Isn’t it possible for Avast to build in a check that it has started (if enabled) and to repeat the start command if necessary?
Probably related to the fact that you’re using very old systems. 
My XP Pro SP3 system is no spring chicken, but I have never experienced this behaviour shield issue.
I just wonder if anything else on your system might have any impact ‘MalwareBytes Anti-Exploit, CryptoPrevent, OSArmor’