Avast can’t scan password protected files/archives, etc. it doesn’t have the password, so in general terms there is nothing to be concerned with.
This is used by the Spybot Search and Destroy program to help restore accidently fixed/deleted items. You can periodically clear out your recovery entries in S&D if you are sure that the item will not need to be restored (after a short period without a problem, you should be safe to delete it).
System Restore retains a lot of information (and takes up a huge ammount of hard disk space if you let it) in restore points _restore folders/files. If my system is running find I regularly clean out old restore points.
You can do this by right clicking on the C:\ in windows Explorer and select Properties, Disk Cleanup, More Option (tab), System Restore - Click on the Clean up… button and that will clear all but the most recent Restore Point, you will get a couple of confirmation as you click Yes and Ok on your way out.
This should free up some hard disk space and stop some of these “Unable to scan: Archive is password protected” entries.