Warning: Showtime added new data to The Sims 3 Downloads folder. This data is contained in a folder in Documents\The Sims 3\Downloads which is named with a number (your user ID). The data stored in this folder pertains to you in-game achievements and SimPort progress, so deleting this folder will cause you to lose all progress with those.
Go to Documents \ The Sims 3 \ Downloads and find the folder named with a number. Make a backup of that folder before you do anything. If you use the Factory Reset method for different challenges, you might want to move this folder around whenever you use a different The Sims 3 folder, so you can keep your progress.