If the head of the screw is raised a tad you can grab onto the outside of it with a pair of linesman pliers and twist gently.
Be sure the edges of the pliers are equal and aren't near any components. You will have to get close to the board if your going to attempt this
so. BE AWARE you could damage if you slip off.
http://toolmonger.com/2007/10/30/dealmo ... -over-100/(i usually use a small version of this)
I've done this plently of times and never caused any damage to the PCB.
Press the circuit board down gently with a little bit of pressure with your left hand near the top of the screw.
(this moves the board away from the screw and removes pressure against the head.
Secondly Grab onto the head very snug and twist counter clockwise. BE VERY CAREFUL. It works if you have no other method.