I have SP1.
I tried downloading direct from Microsoft and installing the Visual C++ 2015 Redistributable (x64) - 14.0.24210 but it failed. Turns out, the likely culprit is the Intel Rapid Storage Technology driver. About two years ago -- at the same time as Microsoft released the rogue update that falsely accused Win 7 owners of running non-genuine copies of Windows, and which rendered such systems unable to update Windows -- I had upgraded my hard drive from 500GB to 750GB. Apparently, the old IRST cannot handle the larger drive, so certain files don't get loaded, which is what has allegedly been at the heart of the problem, when all along I thought it was the rogue update.
This, after 7 hours of hunting, testing, hunting, testing...
But when I use Intel's Driver Update utility to scan my system so it can tell me which driver to download, it says no can do, your is an OEM and has likely been modified by your computer manufacturer; contact them. I call Asus and their tech guys don't know which driver to give me because I upgraded by hard drive.
So now I have to call Intel. Nothing whatsoever to do with Sigil as it turns out, though everything to do with Sigil because without it I cannot build the ebook I am in the middle of writing.
So back to the races. Intel's Pacific offices are now open.
Thanks for the response, though.