To make a new clean game in My Documents - so that you can safely install a new Expansion Pack or Stuff Pack, apply a Patch or use Create A World, or maybe just start a whole new game with a whole new set of downloads, for example for themed play.
Be aware that this takes up quite a bit of space, so you need a large-ish space for your My Documents folder. (My whole My Documents\Electronic Arts folder as pictured here is currently 8.3 Gigs.) If your My Documents folder is on the C drive, (as it is by default on most Windows computers) and hasn't enough room, please see this page for how to move it.
Simply rename your Sims 3 folder from My Documents\Electronic Arts\The Sims 3 as above, so it should be named something like The Sims 3 playing. It's quite safe to leave it in the same folder.
Now when you start your game, it will produce a clean My Documents The Sims 3 folder. I rename that too, and simply rename the one I WANT TO USE CURRENTLY to The Sims 3 when I'm ready to start building or playing.