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).
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. So there should be an automatic reaction to delete them without investigation.
You should be able to unlock this file to allow for deletion,
bearing in mind about what I said about burning a copy first and the warning not to delete (wherever that comes from) - Unlocker
http://ccollomb.free.fr/unlocker/ is also good as it also has a few additional features to not only delete the files but stop any process that is stopping you from deleting a file. Youy shouldn't be deleting MS Word if you want to remove a non-essential program you should use add remove programs.
Is defrag correct in its stating that you only have 15% free HDD space and what is the size of your HDD ?
if so you are in need of a huge clean-up, here are two capable of getting rid of temp files and other associated debris -
ClearProg - Temp File Cleaner or
CCleaner - Temp File Cleaner, etc.. When your HDD gets to within 20% I would say you should be considering a new larger HDD or second HDD and move non essential stuff to that HDD or archive data off it to CD/DVD to clear space.
