When it happened to me, I think that it didn't happen at the same point, as it could happen just beginning to scan (the first files in C:) or a bit later.
But it is possible to scan that files if I scan them, choosing the option to scan files (and then checking C:, eg.).
When the Window pops up, the skin of Avast seems to be going on checking files, you can move the mouse, to close the window, or to send a report to Microsoft, though it is a window from "virus scan".
I send a copy of the window.