How do I factory reset sims 3?

So I purchased the sims 3 a while back and I loved it. I have been playing the sims 4 lately but wanna go back to sims 3 and play both. But I remembered my game was really messed up. It worked 100% normal at first. I started downloading CC, and that's when it went downhill. So first problem is in CAS, you know when you put your mouse over the sim your editing and it will highlight that area? Well the bottom part of my sim is Highlighted all the time. And when I put my curser on the sims head, it will highlight the head and the body will go normal, but once the curser is off the head the body is back to being highlighted. I hope that makes sense. I have tried so many things but I can't seem to fix it. The sims looks a bit odd too in the world. Everything looks great and clear, but the sims look pixelated or somthing and not clear. Now mind you when I first started playing, the sims looked great. Anyways the biggest problem is for the past 3 years ALL of my pets are completely black. In CAS they are black and in the world is the same. Some random animals that go by in the game that I don't own have Color, but if I go to edit them they will turn black. I know it's not my computer because everything was fine before and it worked great. I have tried so many things over the years and I am done with it. So I want to do a factory reset. I looked it up on google but it seemed to technical cuz I'm not very good with computers. I don't care if I lose everything, I would rather start fresh tbh. So could someone give me step by step simple instructions on how to factory reset? And I don't have all the game codes either idk if I would need those. I have quite a few expansions too. And please try to make it as simple as possible lol, I am only 17 and I am * when it comes to computers. Thanks