Just an update, for what little it's worth... I'm still using Avast Free.
I don't like it much -- occasionally it's clearly doing something
behind the scenes because the computer becomes unresponsive, but it's hard to see what. One of the more positive points of Comodo Free -- however sluggish it might have been in comparison -- was that you could go and look at the tasks it was running and, if appropriate, tell them to stop. Avast, in comparison, is positively secretive.
I guess I have no right to complain about the adverts it throws up, but (and I have no real evidence for this assertion) it feels like it decides it's going to put an ad on the screen come what may, whatever else might be going on, because responsiveness will just go away entirely for a few seconds, then an ad will appear, then it'll be nicely whizzy again.
A side-issue: I've never really felt comfortable with Readyboost but I bought one of those tiny Sandisk things that hardly sticks out of the USB socket at all, 8Gb, reformatted it to exFAT (having seen a suggestion elsewhere on DC) and gave it all to ReadyBoost and it actually seems to have done something useful. The netbook is still occasionally sluggish but when Avast isn't busy doing anything in the background it actually feels faster than it's ever been. So... well, we'll see. I'm still open to suggestions!
Oh, and by the way:https://addons.mozil...addon/ublock-origin/