Avast in boot scan mode-how would you set that up?
Right click the tray icon>Open avast user interface>Scan computer>Boot time scan and then schedule it. Set it to examine whatever you want it to. Restart the computer. It could take an hour or more.
I would do this
only if there is malware detected by Avast, that investigation indicates is the real deal (rather than a FP), and normal removal doesn't work.
There is little point in doing it if Avast doesn't detect it. If it is run, and it turns out it is a false positive, too late. The file will have been quarantined. Worst case scenario, it's an important system file. This is a worst case scenario, possible but not very likely.