Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned, so you don't want to remove them just because they can't 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, 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.
If you can give some examples of those file names, the locations and reason given why it can't be scanned might help us further ?
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You can view the scan results again, Scan Computer, Scan Logs and select the one related, you can give some examples from that or attach an image (screen capture) of it to the post. Use the Attachments and other options, link in the Reply window.
Or
If you have the generate report file option enabled in the scan you can view the raw data in the text file for the scan you did, e.g. C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report\Quick scan.txt (XP location) C:\ProgramData\AVAST Software\Avast\report\Quick scan.txt (Vista, Win7), or whatever the scan name was.
