Just been watching one of Hawking's docs, he is discussing time travel, and he mentions the paradox about if you invented a time machine and then went back in time and somehow managed to kill yourself before you managed to invent the time machine. He states that this would create a paradox and is likely impossible. If you think of the universe in a linear way, it is impossible, but if you instead expand the way you see it, and imagine the universe as a series of moments then it is quite possible to go back and kill yourself and not create a paradox. The way you do this is simple, you are Person (Bob) A in moment A, you decide to make a time machine, Person (Bob) A creates time machine, Person (Bob) B, C, D etc also makes a times machine but not quite the same as you, they are you but in their moment something however insignificant changed their perspective. Person (Bob) AAAB1001, might have never even wanted to make a time machine he took a different path in an earlier moment - hope that makes sense. Anyway back to Person (Bob) A, his decision creates the time machine and he decides to go back in time and kill himself. Or someone he perceives as himself, but when he goes back the person he sees is not Person (Bob) A but another Person (Bob), maybe he is Person (Bob) B, or Person (Bob) ZS1, they also exist in a different moment and a different perspective. Person (Bob) life changes through every moment, and every moment Person (Bob) splits into many Person (Bob)'s, this happens constantly and every consequence of every moment of every Person (Bob) is played out. So it is logical that in one of those moments, Person (Bob) A can go back and kill Person (Bob). I'll try to make it make more sense when I think about it more, but I just wanted to get it down whilst it was fresh in the mind.
***Addition*** 23/09/2010 09.00
You have to think of the life or reality of Person (Bob) has always now, we generally imagine now as a progression forward, we create this forwardness because we have created time. Since we have created time and time moves forward, we perceive our existence as past (older moments of time) present (now moment) future (moments yet to come). This is incorrect, time does not exist, it is a human creation to help us form order out of chaos, it gives us the illusion time moves forward. So back to the Hawking's Paradox, we are always in the present moment even if we are capable of creating a time machine. The time machine will take us forward or back in time but it will still be our present moment. You don’t loop back and re-engage the old moment again, that has gone, you see a new moment, but you perceive it as the past. You are never going to see a time traveller from the future ever, you will never see a time traveller from the past either, for that is impossible, in our moment of perspective.