After leaving Avast! in what I thought was a well-behaved mode and successfully booting up my computer a few times, I ran into another freeze-up exactly like the one a week ago. The Desktop was about to be displayed, the "Microsoft Sound" started to play, and clunk! -- black screen with a green bar across the top; mouse cursor frozen in the middle of the screen; no response to the "Windows" key or to Ctrl-Alt-Del. I waited 60 seconds, then restarted in Safe Mode and handcuffed Avast!

I'm still going to use Avast! to scan with from time to time, but not let it start itself during bootup any more. As I recall, anytime I run it it tries to make itself self-starting next time and I have to follow after it (like following an elephant with a shovel, if you ask me!) and restore msconfig to the way I want it.

My WinRAR registration key is still there, so maybe that part wasn't Avast's fault anyway.