all antivirus programs have realtime protection that monitor evrything that goes on inside your computer, meaning it will detect a infection without you have to do a scan (not dormant malicious files)
Many files in the computer can be protected by the OS or the program that they belong to or it can be files you have zipped and password protected.
When avast can not scan a file, it should say why in that message
If the latest as soon as you unzip your password protected file, avast scan it (you dont see it) evry time you save, open a doc, download a file, move a file, open close a folder .... avast scan
Wikipedia >
https://en.wikipedia.org/wiki/Antivirus_software==============================================================================
Real-time protectionReal-time protection, on-access scanning, background guard, resident shield, autoprotect, and other synonyms refer to the automatic protection provided by most antivirus, anti-spyware, and other anti-malware programs. This monitors computer systems for suspicious activity such as computer viruses, spyware, adware, and other malicious objects in 'real-time', in other words while data loaded into the computer's active memory: when inserting a CD, opening an email, or browsing the web, or when a file already on the computer is opened or executed.[97]
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