For a start, it would help to know which avast you're using (home? pro? some other?). I assume if you've "just downloaded" then you've got the most recent version of whichever.
With extremely rare exceptions, the only time I get a "can't scan" is files, typically Win system files, which avast says are in use by another process. Oh, and I did set up one don't-scan exception, a set of folders which I know are nothing but a huge collection of JPG's (images), not worth the extra time they would need for scanning. For some reason avast insists on reporting those as "unable to scan" too even though they're right in its own exception setup.