Alright, so those of you who remember my "Fate exists?" thread will probably notice some similarities, but bear with me :)
Let's get right to it with an example where most people would say that a decision has been made, like when some guy decides to rob someone for money. Now, if you were to reverse time again and again (it could be any timespan from years to minutes), do you think that (a) this guy would have decided to rob this person after each time reversal? (As supported by determinism.) Or do you think that (b) each time reversal would have resulted in him making different decisions each time? (As supported by quantum theories.)
Those are really the only two possible outcomes that comes to my mind, and this is where the problem starts. If he decides to rob the victum every time, then that suggests that he couldn't possibly have made another choice at the moment, meaning that he has no free will, but follows a predetermined pattern that has already decided all of his future decisions.
If the outcome would be different each time, on the other hand, wouldn't that too suggest that he has no free will? If each and every decision you make throughout your life is based on randomness then they might as well have been completely different, suggesting that you never really had control of any of your actions, and thus no free will.