Guys - thanks for your help.
Linney, the main symptoms are:
1. For quite a few months, inconsistently, I've gotten the Blue Screen of Death on shutdown. I've seen so much of this that I now pay little attention to it. I know - probably a bad idea. The next time I start the computer, it always starts up o.k., except as described below.
2. Recently, AVG has started treating AutoHook, a very helpful utility app I use, as a virus. AutoHook loads on boot, but does not do anything until you are running AutoCad. AutoHook is a very small key macro program. It's used to program keys to send strings to the console. For example, I program the num pad Enter key as: "copy <enter>". AVG decided it does not like AutoHook, and disables it, apparently by removing the .exe file. When the computer can't find the AutoHook .exe file, I have to click through a few Windows notifications, basically saying; o.k., never mind, forget about AutoHook. AutoHook installs itself so it loads on boot. I'm sure I could set it up so it just loads right before AutoCad, but I doubt this would fix the problem.
I tried removing and reinstalling AutoHook several times. Usually works o.k. for a couple of days then goes back to the same problem.
I took my hard disk to a professional I know and trust. He did a scan and spent an hour or so with all kinds of malware removal tools. He found a lot of problems and fixed them. Computer worked great for a week or two, then the BSODs came back, as did the problem with AutoHook. I'm inclined to think there's a recently created virus out there that somehow looks a bit like AutoHook, and when AVG does a scan it mistakes AutoHook for the virus.