I went into regedit, renamed "Sid Meier's Civilization 4 Gold" so that the "Gold" was no longer there, then added a new key, so that I now have the following:
HKEY_LOCAL_MACHINE\SOFTWARE\Firaxis Games\Sid Meier's Civilization 4\1.74
This is the only thing under Firaxis Games, other than a couple of other files under "Sid Meier's Civilization 4", and removing the word "Gold" does change things: now when I try to install BTS, it tells me that a newer version of Civ is available, which it never used to do, and then the patch fails to install, causing the installation to quit.
I also tried leaving the word "Gold" in there with the new 1.74 key, and in that case, installation continued as before, until the good old "Catastrophic Failure." Is there something else I'm missing here?
Thanks.