Using the forum search function for your topic title would no doubt have returned many hits as this is a frequently asked question.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.
Many programs (usually security based ones) password protect their files for legitimate reasons, 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, but there are many reasons why files may be password protected.
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.
If you can give some examples of those file names, the locations of these password protected files ?
