Sorry, my description was incomplete.
I had already tried other resolutions, full screen/windowed, vsync on/off, clean Windows start, driver tweaks etc.
The display stays garbled in-game. When I get to the city map screen it's the same, though in different colors.
The video driver re-install I haven't tried but there's no reason to believe that it will make any difference, as all my other games work fine. Also, diagnostics and Futuremark all detected no problems.
So thanks for the help but I'm giving up on this. I'll save this game for the new PC...