This is different from a total-game reinstall - This sets the game back to when it was first installed with a fresh new Sims 3 folder.
Here are some simple steps to help with backing up and restoring Sims 3 Store Content.
This guide gives step by step instructions on how to download content from the Sims 3 Store.
This article goes over how to save a lot to the Library and Share or Export them.
This little how-to goes over the basics of creating a blog post on our site.
This article goes over how to save a household to the Library and Share or Export them.
If it's been awhile since you've saved a household, house or community lot and it's sitting in your Library folder, you can view the files with S3PE.