Oh wow, thank you both for all the help!
I'm not so great with technological language, so let me try to understand little by little:
I'm using Mac... I know, it's not the best for Sims, but well, it's what I have. In Mac instead of going to "My Documents" I'll go to "finder", the rest is the same to find the Sims 3 folder (documents/electronic art/The Sims 3).
So I tried to do the factory reset as in the link: rename the folder to "the Sims 3 OLD", started the game, exit the game, then there should be two folders... no, there's just this one named "the Sims 3 OLD". I expected that when I had renamed the folder and started the game it would start blank, but no, all my recent games were there, its as if everything is saving in a virtual folder, I don't know if that is possible...
I first bought Supernatural, that is a CD; then I downloaded Ambitions, but Ambitions was released before. I loved the idea of downloading expansions, I was thinking of downloading Island Paradise... do you think that would hurt my game?
I haven't registered my games to the site, I don't know why but the time I tried I couldn't... and my CD had a limited edition content pack, there's a page I should visit to help me with that.
What happens with the game I downloaded it I unistall everything?