In every sci-fi movie or book that deals with time-travel they always talk about "you can't change anything because everything here will change." This is stupid for two reasons:

1. As soon as you set foot somewhere something has changed. You stir up dust, you alter air currents, you change the oxygen to nitrogen ratio. See the Butterfly Effect for more on this.

2. You cannot change the past. This is for the simple reason that the past has already happened. If, in the future, I were to go back and try to kill my parents (not that I have anything against my parents) I would be unable to do so. The past, which I was trying to kill them in, has already happened: I would have already killed them. And obviously I haven't. If people ever travel in time, nothing will change, because everything would already have been changed.

The only exception to the universal rule of incorporating this rule in everything sci-fi is Michael Crichton's book Timeline. However, his rule was also wrong, that no one could change the past simply because one person couldn't do anything big enough to change it. Anything anyone does changes the future. Again, see the Butterfly Effect.