Very tentatively, there may be an improvement to the computer situation. I remembered that the very first programme to start glitching was Search and Destroy, and that Search and Destroy was glitching even in Safe Mode, and that, after the hard reinstall of XP, S&D was one of the programmes I reinstalled before the computer started glitching. I also realized that, given that Yahoo's anti-spyware is part of my BT package, it runs in the background all the time. And I know that S&D is persnickety - it sometimes lists other anti-spyware programmes as spyware and makes quite defamatory remarks about their proprietors having stolen code from S&D. So, I deinstalled its ass and then removed it wherever I could find it so that it is gone, gone, gone mofo.
And guess what? Only one glitch of any kind in the last 24 freaking hours and that when I was running a whole load of programmes simultaneously. I can watch my shows again without having to wait for reboots every few minutes. At some point, I may try playing Civ, not because I want to but just to see if I can.
All this, and it was a programme clash. I am so stupid not to have thought of that earlier.