Thanks, JWA. Uninstalling Avast! worked. I uninstalled Avast!, enabled Windows Defender, installed the definitions file update, disabled Windows Defender manually (from the Administrator option on WD's Tools tab), and re-installed Avast!. I will be interested to see if I encounter this same problem again, if Windows Update tries again to install a WD's definitions update.
Pondus, before JWA offered his suggestion, I was preparing to generate and post the log files you had recommended. Of course, malware is/was a possibility, but the reason I was skeptical of that possibility (it was no where near the top of my list) was twofold. First, as I indicated, I encountered this problem simultaneously for the first time on two similarly configured Windows 8.1 laptops -- but I don't use the computers for the same things, so how would they both have acquired the same malware at the same time (unless it was embedded in some major piece of high-reputation software, I suppose)? Second, others on the net have reported the same experience (linked in my previous post). Now JWA has reported his encounter with this issue as well. I hope Avast!'s programming engineers take a look and can track down the source of this behavior. As I indicated in my initial post, it is possible that it relates to something MS did differently when pushing out the recent WD's definition file update through Windows Update.
I love Avast! despite one or two inevitable hiccups over the years. Nothing is perfect, so perfection is not an option. But Avast! is great.
