To try to keep a long story short:
Upon downloading The Sims: Seasons through Origin, our installation of The Sims 3 has apparently become corrupt and unplayable. The game will not launch, and all of the code/data in the game directories seems to have vanished. We have given in, and decided the only solution is a complete uninstall and reinstall through Origin (to make sure we get the digital download of Seasons that we purchased today along with all of the expansions we have on disc). We lost the booklet containing our registration key for The Sims 3, but managed to find it through regedit. Before a complete uninstall, I tried to register it on the Sims website (thesims3.com) which failed, and again through Origin which reported that the key is already in use.
I'm not sure how to uninstall a game that refuses to open in the first place, but before we can worry about uninstalling we need to make sure the product will install via Origin first, and I'm sure it won't work if the key is "already in use" by someone else. It is possible that we registered the game under a different email account when we first purchased and installed the game, but if we did, there's no telling what email account we would have used. Our expansions for Late Night, World Adventures, and Ambitions have all registered in Origin successfully, and the registry keys found in regedit for the expansions match those in the booklets that we still have.
How do we ensure we can install the core game The Sims 3 through Origin?
Thank you for your help.
You should have the icon of the expansion in your Origin client. That icon will not go away when you un-install the expansion. That's one of the big advantages of Origin, you will always have your games there. You can always re-install your games and expansions from there.
When you purchase The Sims 3 or any of its Expansions or Stuff Packs through Origin, you no longer need to manually register your product through The Sims 3’s website. Just make sure that your Origin account details match the details of your Sims 3 account, and Origin will register your products for you.