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I did look at the active processes in Task Manager and there were only the usual 2 or 3 processes which were taking up even 1% of CPU time.
Perhaps you can tell me which columns amongst all those that you can display have any bearing on active windows because none seem to have anything to do with that at all.
But thanks for reminding me about Process Explorer - I have it already and because I seldom use it I'd forgotten about the button which lets you click on a window to see which process is responsible.