A screw won't unscrew, how to get it out?

I wanted to take out an old hard drive in my computer but one of the screws won't unscrew. I can keep screwing it eternally to either side, but it doesn't come out. Any tips on how to get it out? I don't have that many tools, pretty much just the one screwdriver, so some way without using tools or something cheap I can buy to get it out would be nice ;)