Sims 3 won't launch, Service initialization failed (0x065d0274) error

Marshmallowycat  Try playing in a new admin Windows account.  Make it local, as in, don't link it to your Microsoft account or any email address.  You'll be able to (try to) launch Sims 3 without reinstalling anything.

If that doesn't work, I'd suggest doing one more clean uninstall and reinstall using that account AND with your antivirus disabled.  As long as you don't do anything else at the same time, your computer should be safe, and you can reenable the AV as soon as you've tried to launch the game once.