Follow up:

Carrara does not uninstall well ... it leaves a whole slew of folders and orphaned files.
I manually went in and removed all traces of it I could find after uninstalling it.
I then also did do a registry scan and fix (with CCleaner). 
It had left a bunch of orphaned registry items as well, which CCleaner fixed.

After all this, I reinstalled Carrara for the umpteenth time, as administrator.

And it STILL crashes instantly on launch, leaving no error message or helpful info as to what is wrong.

I'm really fed up after a whole day of trying to fix a program that was working okay then just quit with no explanation whatsoever.