Hi again Mike. I completely uninstalled ZoneAlarm (entirely unapplied toolbar first) and activated the Windows firewall. I'm sorry to say that within fifteen minutes the mouse was doing exactly the same as under ZoneAlarm.
One thing that I hadn't posted was that before making this change I noticed that on occasions, while the mouse worked apparently correctly within the page in use, it completely failed to have any effect if any button on the taskbar (including "Start") was clicked. When returned to the page in use, it went back to working correctly once more.
Because I am unsure whether or not my Spanish "Telefónica" (national telephone company) router includes a hardware firewall or not, I would feel more comfortable (rightly or wrongly) reinstalling ZoneAlarm once again and deactivating the apparently one-way Windows firewall, even though I imagine that ZoneAlarm will have to begin its education process afresh. Don't hesitate to tell me if I am mistaken.