1. you have a antivrus with realtime protection, meaning every file that moves when the computer is on is checked
so a quick scan once a week is plenty. in the scedule settings there is also a wake up setting...
or you could just activate the screensaver scan, the it scan everytime you turn your back to the comp and the screensaver start then you dont have to think about doing it
2. bootscan is something you use if the normal scan have problems removing something it find, or if avast tell you to do it