crinrict, thanks for your attention on this matter. I don't have bitDefender installed (actually until you mentioned it I wasn't aware it existed). If it causes more issues I'll keep that in mind and let others know.
I have managed to get my game working. It's been a haphazard trial but it's done. While I know how to make my copy work I don't know what's causing it.
I tried the steps outlined in the page you linked. No joy. I checked for outdated drivers. All up to date. In a fit of pique I even tried uninstalling ItF. Nope.
Then I tried uninstalling NVIDIA GeForce Experience 1.7 (which wasn't updating to 1.8 properly). Then I installed GeForce Experience 1.8 - the updated and most recent version of the program - as found here. It installed flawlessly and I scanned for games, noticed a 'game-ready driver' wasn't at its most recent version, started it downloading. Then I went to the Games panel in that program, selected The Sims 3 and hit 'Optimise'.
On a whim I started the launcher - ItF was not installed and neither was the updated version of the game-ready driver. But the game started. It ran without a problem. I restored my user data and tried again - still no problem, though it did drop my resolution back to 1024x768 (but that was easily changed back in-game).
So. Fixed, right? Not quite.
It won't run when I load the Launcher via Origin or via a Start Menu link. The Launcher loads without a problem but not the game itself. When I run it through the 'launch game' button in GeForce Experience it works every time. Which to me seemed really weird. So I bypassed the Launcher entirely and ran it via TS3W.exe - nothing. Not a blip.
And then I right-clicked it and chose 'Run as administrator'.
Bam. Worked perfectly. And it also works perfectly when I run the Launcher file 'as administrator'. I assume that GeForce Experience is forcing the Launcher to load with administrator privileges, but the Origin client isn't. Either way it works. I'm not sure why it requires admin privileges now when it never did in the past but if that's the only issue I can work around it.
Note to self: the moral of the story is to try running with admin privileges first.