one theory of time that I've always like is that you can't go back into the past and change it - cause you'd actually be carrying out what has already been done.
I.E. You go back into the past to prevent Bill Clinton from reaching the presidency, but then you find out - Bill was in the presidency BECAUSE you had already, in the past, attempted to stop him. You had instead, CAUSED it inadvertently.
Under that theory there is nothing you can do to change the past - cause you've already tried. heh. I wonder if this is a little too deep for some of you.