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Many programs (usually security based ones) password protect their files for legitimate reasons such as AdAware and Spybot Search & Destroy, there are others (and avast doesn’t know the password or have any way of using it even if it did know it).

When you run scans with the above programs and you delete harmful entries that they detect, a copy is kept (in quarantine/restore/backup) in case you need to reverse what you did. These are usually password protected, you should do some housekeeping and delete old backup/recovery/quarantine entries (older than two weeks or so), this will reduce the numbers of files that can’t be scanned.

By examining 1) the reason given by avast! for not being able to scan the files, 2) the location of the files, you can get an idea of what program they relate to. You may need to expand the column headings to see all the text.

Files that can’t be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.

Decompression bomb is basically a file that is heavily compressed so to unpack it could mean a very large file, that may be unmanageable. This was/has been used previously to crash a process making the system vulnerable. There is no way to confirm this so avast just reports this as a possibility but doesn’t unpack it to scan.

Empty the recycle bin and see if that clears it, this however, is starting to become more prevalent for viruses to place elements in the recycle bin.