No exclusion is necessary just ignore them.
You don't say what the file names are or what type of scan you are doing?
As by default avast doesn't scan archives as they are inert until you extract the content and run and files, before that happens the real time shield (file system shield) will have scanned them.
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). So essentially there is nothing you or avast can do about this.
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.
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 and if it appears a legitimate purpose. You may need to expand the column headings to see all the text.
To exclude you would effectively have to exclude the folder, which would be introducing a hole into your security or have to exclude every password protected file (hassle).
