Take your screwdriver and place it in the screw.Take a hammer and safely tap your screwdrivers handle and give it a few good whacks(safely!) and then try and turn the screw,repeat a few times.This should help free up/dislodge any rust that is making it hard to turn.If this fails, get the drill out and drill the head of the screw off using a bit that is around the same size of the head.I have used both methods to do this exact same job,both with success.

David