is the HD bad? are you reusing it for another PC?
If possible you can leave the HD inside the case, usually there are lots more places in a case than just one spot for a HD. Change the dip switches to slave and the new HD to master, have extra HD space if its still good.
If it has to come out then maybe try a different screwdriver, usually a larger one will work. Another method maybe a hex bit inside a 1/4 in socket with a ratchet, sometimes you can get a good ammout of weight behind that ratchet and keep the bit from backing out of the screw head. If all else fails there is always vice grips, and a drill to knock the head off the screw, keep an eye on metal shavings if you have to resort to the drill bit.
RHIN0