sometimes impact can help with overtightened screws. i would have a friend tap (lightly) on the top of the screwdriver while you turn it and while pressing down really hard on it. sometimes heat can help loosen things up. hold a soldering iron to the top of the screw head to heat up the screw. try at your own risk. these methods i've used on cars but never on computers. the dremel idea is one to consider but really risky due to the mess. i've also heard that tightening it further can break it loose. (sounds crazy but it can work.)