I posted this in the official forums, but have so far had no real help, so I created an account here to try my luck with you fine folks. I did see there was a similar posting already, but that person's issue involved switching from full-screen mode to another program, and the crashing behaved differently for them.
For the last several (realtime) days, I've been running the Sims 3 (with Supernatural, Seasons, and University Life) just fine in windowed mode on my Windows 8.1 desktop. Two days ago, I created a new family (based on my D&D party) and put them into a large house from the premade selection menu thingie in Sunset Valley. I got their house set up and went to bed. The next morning, every time I would leave the game window to its own devices and click away to Firefox or message windows or anything, I'd return to the Sims to a "The Sims has stopped working" and the game would shut down. I saved the house with the family as well as the family without the house to the export folder and then told Steam to delete all local content. I then removed the folder in Documents and set Steam to reinstall the game while I went to work. I re-added the family with the house as an experiment and tried the game, got the same issue. I was able to play the game with no issues in fullscreen mode as long as I didn't alt-tab to do something else, but for me, that's boring. This morning I moved the family to a new house and bulldozed the other one then restarted the game, still had the issue. I deleted the family from the main menu, deleted the saved family and saved house in the place sims menu thingus, restarted the game, created a new sim from scratch, and still had the issue. I'm thinking I'll have to reinstall the game one more time, but that's a lot of work that I don't want to deal with right his minute, so if there's anything any of you folks know or can think of that I can try, I'd greatly appreciate it.
I have no mods or non-developer things beyond running the game through Steam, which I've done for years and never had an issue. I will happily provide crash logs or whatever, assuming someone can tell me where to find them...
ETA - This machine is just about a week old, and has 8g of RAM and a NVIDIA GT 640 video card, which should be more than enough to handle this game, as my old Compaq desktop with its integrated chipset graphics and 3g of RAM had no trouble playing in windowed mode. I'm also able to play other games (so far I've only got Star Wars: The Old Republic installed on this machine) and have been able to tab with no issue whatsoever.
ETA (again) - If it matters, the game continues to function normally after I've tabbed to another program, it's only when I click back to the sims that it crashes.