I can also confirm that the backup.exe is the core of the issue now. In order to boot into a computer, I boot normally, with the ethernet cable unplugged, forces local login only. I get enough time to get into task manager and kill the avast backup process. Computer stays on no problem, even after booting it up. Going to rename the file as suggested, and then install the latest client.

Very unnecessary headache in the midst of ongoing summer upgrades!