Brian Greene has a great explanition of time travel in his latest book. "The Fabric Of the Cosmos"
Basicly he says that there are 2 stances you can take on time travel that are not rife with parodox. The first stance is that when you travel back in time you merely travel to a paralell universe that is similar to you own. If you go back in time and attempt the cleche' killing of your own father, you can go ahead and kill him with no reprocussions because he is merely one of an infinie number of identical fathers. A parallel "you" will not be born in this universe but you (the one that killed your father) won't wink out of existance because you still have a father back in your own universe.
This is a very unristrictve kind of time travel in that you can do whatever you want and then see the reprcussions without worrying that you will wink out of existance. Remember you can travel forward in time just by accelarating so that if you wanted to see what a world would be like without you in it ala "it's a wonderful life" all you have to do is hop in your space ship and travel at a nice relativisitc speed.
The second interpretation is much more restrictive but still doesn't allow for paradox. It is my personal favorite becase it makes for good short stories.
In this interpretation of time travel When you travel back in time you DON'T go to a parralell universe. There is no need for multiple universes.
So what happens when you go back to kill daddy? You can't. Why not? Becase you exsist! and therefore couldn't have traveled back in time to prevent your own existance. The paradoxes come from the misconception that when you travel back in time, time "starts again" from where you arrive. But it doesn't! Everything has already happend.
If in the future you wind up going back in time, you are ALREADY there! When you travel back you can't "change things" to make a "second past." That past already exsits WITH your changes. Space time is then seen like a frozen river or a loaf of bread (Greene's term). If we could look at a time traveler outside of space time we would see his/her whole trajectory through space time as one solid unmoving shape.
I like this second intepretation best even if it does imply a kind of fatalistic deturminisum.