For what it's worth, when you boot up 98SE in Safe Mode, there is no problem accessing or exiting the GUI, and I was able to do the few things I tried off of the GUI menu without the system freezing up (although in Safe Mode a lot of avast options are grayed out). So there was no problem initiating a scan and you can set the level, but in Safe Mode it's too slow a process to be practical.
Back to "Normal" Mode. As a point of information, I disabled GoBack (Roxio 3.2 GoBack Deluxe), and it had no effect on the problem, so if there is a driver conflct, GoBack is not the culprit, at least if my experience is typical.