I get the same message about Avast could´nt scan certain files. This occured after I by misstake downloaded Windows 10 and somehow the installation failed. After that I get these messages about avast could not scan some files. I can see that the files it can´t scan is files that have something to do with windows 10. I saw that you downloaded files with windows update maybe there is a connection there ? This is just a thought have no idea if it´s relevant to your problem.
