As a developer I frequently need more power out of my system. So I'll turn off many programs and services including all four of the Avast shields. I use the tray context menu to "Disable until computer is restarted". But in the last few releases (months?) after a full shutdown, power-off, restart, Avast shields don't come back on until they are manually enabled.
Windows 8.1 Pro with free Avast 17.6.2310 build 3625.0, defs current as of today.
Only using Shields for File, Behavior, Mail, Web.
HTH