Time machines are a paradox. Take the example that you can go forward into the future and see yourself. You would then travel back to present day, and most people would try to change their future. But that means that the future they saw could never exist because they just changed it, or that the one they saw was the changed one, but then that couldn't have been because they had not yet seen it to go back and change it.

Also, if you went back in time, you would definitely kill something or other that would change something, wether unimportant or extremely important in your present day --> you are billions of years (they say the Universe was created 15 billion years ago) from home, which means the results are amplified more than, say, if you went back just one year. There are too many things that we don't know about, that we ca either kill or change, even if we can't see them, to be able to garanty(sp?) that we wouldn't kill or change anything.