Having removed it and AVG (for the time being), my system is back to the "kinda pokey but working" state it was in this time last week, which at this point I'll take as a victory. The last item that remains is to remove MalwareBytes and reinstall AVG to compare that result.
As for Spinrite, moving to a SSD, and all that jazz, that can all be dealt with later. Right now my priority is, with the computer working again, to get myself on track for AC. That has the tightest deadline.
Thanks for putting up with my crankiness, all. We have moved back down from Red ("Brain About to Explode") level to Orange ("Too Much Crap to Deal With, Dammit") mode. If I'm lucky, I can get down to Yellow ("Gah!") by the end of the weekend and hold there for at least a little while.