Thanks for the information, everyone ....
I think what I'm looking for is a Windows installation whereby LilyPond's python scripts are run by its own python.exe, while all other python scripts are run by the system's python.
This is likely not possible under Windows. *sigh*
However, since the python script I use most (midi2ly) doesn't work on Windows anyway (fortunately, my main system is a Linux box, where everything -- more or less -- just works), I think I'll just kill LilyPond's registry key and see what happens ...
Regards,
Fred
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“To my knowledge I have no knowledge.” -- John Kerry, the Secretary of State of the United States of America, in response to Senator Barbara Boxer, 3 Sep 2013