Marshmallowycat Did you uninstall or deactivate any packs you'd bought? That's the usual cause of this error with a Steam install, and yes, the registry entries are usually the cause. Did you know where to look for them, or did you search? I ask because they're in two different places and listed as Sims(Steam), so a search for Sims 3 might not turn them up. They're specifically here:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Electronic Arts\Sims(Steam)
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Sims(Steam)
If this isn't the issue, I'd suggest doing a clean uninstall and reinstall of Sims 3 (not the EA App, that's irrelevant here), as described in this post:
[Read First] Running Sims 3 in Windows | EA Forums - 8323446
The clean uninstall, and reinstalling in a different location (and you can create a new Steam library if you want), should address more issues than a simple uninstall.
I did two clean unisntalls and reinstalls of The Sims 3, one with CCleaner and one with Revo plus both times i went into the registry editor and deleted all the files that had sims or sims 3 in their name and also deleted the recycle bin. It didn't work, I still can't launch the game
