Hi, There are some special definitions for boot scan alone. For that you need to download separate definitions files from avast.
The boot scan is performed when the system does not have any service loaded. If the virus , is found, it would ask for options to quarantine or delete. I usually run this once in a while, as nobody knows that we are downloading with the original files. Boot scan shows, corruption in the zip file that would enable you to delete it.
The scan would be fast compared to the GUI scan. I experienced an issue of a virus in sound form, which avast did not find in full scan in windows running. I just resorted to boot scan, and it found out and asked me the option. i quarantine. i do not know the exact name, but the virus is downloaded in the form of wave , with a music or video file. Please do the boot scan and then tell your experience