Aaaaahhhhhh! The downstairs machine has a parasite.
Maybe I wished it upon myself by writing so much about (real) bugs.
I didn't know computer parasites existed, until now. What a hassle! It has just about ruined the machine. I tried everything - removing it with uninstall, removing it by deleting the folders, removing it from Startup. What with plug and play, and the complexities of networks, I don't know as much about tinkering with computers as I did in the old DOS or Windows 3.1 days, and I was getting frustrated. I tried several different programs that remove "spyware," most free and one commercial. I got better results, but still not 100%.
I think I'm going to have to learn some new things. Go through the various suggestions and remove the traces manually, as I would have in the DOS/3.1 days. Either that, or format the machine, which I really, really, really don't want to do. On the bright side, while I am trying this and that - and the hours slip away - I have been been doing a low priority but overdue task, which is cleaning up that machine, removing files that are outdated and not needed.