The easiest and safest way to be sure you keep all your created items (lots and sims), recolors, store content... is to copy and archive your complete user "The Sims3" folder. Path is "C:\Users\[username]\Documents\Electronic Arts\The Sims 3" under Windows. Then, you can always put back your things bits by bits after.
If you want to make a more detailed back up:
Store content: You downloaded store items come as .Sims3pack and are found under "Downloads" in your user The Sims3 folder. Archive them too. When you have reinstalled, you can put them back and reinstall them through the Launcher. You might need to create a Downloads folder if there isn't any. Store items can always be downloaded again from you account on the Sims3 website (once you're logged, you need to find your Purchase History under you account settings).
Your created stuff: There should be a "share" (or "export"?) button on all your created or recolored stuffs. In Edit Town, if you click on a lot or on a household/Sim, one of the option is to share. I think this creates a .Sims3pack of your item, probably found under the "Export" folder in your The Sims3 user folder.
Then, in CAS, select one of your saved Sims and look for the same "share" icon. I think it should create a Sims3pack too. Otherwise, saved Sims are saved under the "Saved Sims" folder, maybe as .Sims, no? I can't check now since I don't have any in my own game and folder.
For patterns and object recolors, it's the same. Browse your various menus in game and export everything you can :-)
You will be able to reinstall your stuff with the launcher after.
I'm going to do a complete reinstallation of the computer + game in the next week too, oh joy! If I find anything interesting I'll post it here.