Let's say clearly: if during an avast! scan,
1. avast! freezes or crashes, it's probably avast! problem.
2. The computer freezes or restarts, it could be e.g. a conflict with some other program, or a hardware problem.
3. The computer powers down - it's probably a hardware problem (e.g. overheating). The scan generates a lot of hard disk activity (and is also CPU intensive), which may increase the temperature of your hardware. I suggest to check the coolers in the case.
4. If you get a blue screen, it could have a bug in avast! drivers (but then, there is not a power down).
This is nothing related with password protected files.
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.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.
The 8% is just a coincidence. Don't fix at it, but in the name of the file and folder.