Essexboy, some good news, some bad news, and some (hopefully) better news. The bad news is the problem re-occurred. The good news is the same FRST fix you previously gave me corrected the the problem.

The better news is I think I’ve discovered the cause. Stepping back for a moment, shortly after buying the computer, one of the two built-in USB ports broke–probably cheap plastic. So, for years, I’ve used the one good port, and a USB hub. Apparently, when recently operating without the hub, I accidentally plugged a USB device (mouse or flash drive) into the broken port–and that “did something”. I don’t know why “the something” did this, or how it could be consistent in results, nor did this damage the USB device, but that appears to be the case. (I’ve taped over the bad port to make sure I don’t inadvertently plug into the wrong port again.)

Sorry for the long post, but I’m telling you this in the hope that this might help you to help others with this same problem. It might not be a problem with Avast, or a virus; it might be an odd hardware glitch. An FRST fix may still be necessary, but you could alert them to be careful with USB devices and ports to prevent a repeat.

Thanks again for all your help.