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I enable scanning of archives every time because once or twice in the past Avast has caught an archive file which had become infected somehow - probably something I downloaded deciding to piggy back on my files - and it meant I could delete it with no risk of the computer becoming infected as on each occasion the infected files have been in My Documents when found in this way. As I’d’ backed the relevant files up and had clean versions, there was no problem.
That’s quite true about Avast spotting stuff that’s infected if it runs, but it can’t necessarily deal with trojans - it really is an antivirus which is why the free cleaning tool is available - and it’s better to stop these before they get started if you can.
To answer your other question - way back in May of last year I had a major problem with a trojan file which downloaded an archive file into my Windows System file and which then every time Avast was run caused Avast to duplicate its .dmp archive files. I ended up having to delete these files manually and was very lucky that I could resolve it this way.
I suspect what your “rocketscientist” meant was that some antiviruses may cache files which they scan - I don’t know the full answer on that one and maybe one of the others can answer in more detail.