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  1. basically it is run before windows starts, so I guess some of the logging might not be done as the avast logging won’t be active, hence the use of a text file specific to the boot-time scan. Though I have never spent much time worrying about it to consider investigating it fully.

Not all of the packers can be used in a boot-time so some of the archive types won’t be scanned, also if you don’t select scan archives when you schedule the boot-time scan that will reduce the scanned files.

  1. CHM are help files.
    There is nothing to worry about as avast is only reporting it couldn’t scan them not that they are infected, etc. and t could be as simple as it couldn’t fully open the archive, so it reports corrupt archive rather than 'I can’t open/extract this archive, etc.).