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, the ones you're talking about. Lavasoft stores its 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. Spybot does the same.
In AdAware and S&D, when you fix/remove things it keeps backup/recovery information so you can restore anything that was mistakenly fixed/removed, etc. After a reasonable time your system has suffered no adverse effects, you can get rid of the older recovery/backup points.
This should reduce the number of protected files.
Many programs (usually security based ones) password protect their files for legitimate reasons such as AdAware and Spybot Search & Destroy, there are others (and avast doesn't know the password or have any way of using it even if it did know it). Do you use any other security based programs ?
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.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.
