Can you schedule a boot-time scanning?
Start avast! > Right click the skin > Schedule a boot-time scanning.
Select for scanning archives.
Boot.
If infected files are found, it's safer to send them to Chest instead of deleting them.
This way you can further analysis them.
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.
1. Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.
2. Also, the packaging of the file could have some error, or use a non-standard pattern...
3. Access denied means, generally, that the file is in use by another process (program) and cannot be repaired/cleaned/moved/handled by avast!
4. avast can't scan files that are password protected, it doesn't know the password. There are many legitimate reasons why a file was password protected. For instance, Lavasoft Ad-aware and SpyBot store their data in a password-protected ZIP archives (to prevent other similar tools from messing up with them). It's really nothing to worry about - it's normal.