Like BossBob says, if it is the threads, then see if you can just pull the screw out. I've gone so far as to super glue a screwdriver bit on to the head of a screw to help pull it out.
If the threads are intact but the screw is rounded out on top, then there are a few things to try. First, I would probably use my Dremel tool to cut a slot into the head of the screw to see if I can get it out with just a normal slotted screwdriver. Other times, I've gone so far as to drill the head of the crew out completely so that the part it is hold down can be removed. And then using a pair of pliers to grab the threaded portion and removing it.
Good luck.