I guess I spoke too soon. The problem has returned.
Watched some videos online tonight, and left the room. Upon return, it was at the password screen again. Logged in, checked the Screensaver panel (it showed "(none)", which is what I had set; the box for 'on resume, display logon screen' was unchecked, and ghosted like the 'Wait 1-minute' setting to its left).
I did, however, find this very interesting Microsoft programmer thread about the issue:
http://www.ureader.com/msg/14743674.aspx
:-) So I'm not alone in this one. I don't run the videos full-screen (like mentioned in on that page, in their discussion), I view them in my browser (unless watching a DVD, then I think it's Media Center that's playing one).
I downloaded Malwarebytes' Anti-Malware, and am going to run it (just to see), but my newly reinstalled system is clean by all accounts. No integrity problems shown after 'sfc /scannow', either. I think maybe a video online triggered (set, or reset) something to do with the time-out or screensaver|password. So far, the only solution is to re-apply the "(none)" setting. Just have to do that after every single session of video-watching--though that doesn't really cure it from happening while watching a full-length movie (it annoyingly interrupts the movie).