There are many paradoxes in altering time. If you went back in time and stoped your parents from ever meeting eachother, you wouldn't exsist and therefore you would not have gone back in time, therefore your parents would have met and you would have gone back in time! I belive that actually stoping your parents from meeting would only create a new paralell universe. In this animation you see yourself as the red ball. You come to a point in your life where you make a decision. This takes you to another point in your life where you make a decision. Well as you see you then go back in time and stop yourself from making your first decision. This throws you into a paralell universe where you continue your life as if nothing has happend except you didn't make that fisrt choice.