TBH, you are better off using the Avast Rescue Disk Feature if you want to ensure a new computer is clean before you use it.
-Resident active protection will protect your system(s) from malware at the instant/moment a file or process is run on your systems.
-Simple file scanning will not protect in this way. It can't.
I'd recommend creating a Rescue Disk USB to use. When the Rescue Disk is created, the latest definitions are downloaded and installed onto the rescue usb.