Sigh.
No "answer" needed... this just to save the next person. This, judging from forum posts arises from time to time, but it hit me 14 Aug 15, I hadn't seen it before.
Longtime-stable Windows XP box running Arduino and Norton Internet Security. Suddenly all sorts of problems when I tried to start an Arduino session. Things laggy, wouldn't start, wouldn't give sensible error messages.
Finally traced it to my Norton Internet Security. "I didn't say this", but if you unplug your internet connection for a moment, turn off... for a moment!... your anti-malware (Norton in my case) and then try your Arduino software, and it starts, you've had a reward for the risk you took, haven't you. Turn the anti-malware on again, quick!
Have spent hours with full system scan, update this, update that... As I write this I am waiting yet again for a system restart on the PC affected.