Re: Clean reinstall of The Sims 3, but still logged in to the launcher?

The launcher uses Internet Explorer to store login data.  If you want to scrub that:

Open Internet Explorer

Click on the little cog in the upper right

Choose Internet options

Choose the Advance tab then click the button that says Reset

Tick the Delete Personal Settings Box then the Reset Button

Close Internet Explorer 

Open the Launcher & login again

I don't see why it would affect your game after a reinstall but if you want to be sure everything old is scrubbed this is the way :c)