Underneath the front bezel, push up on the two small tabs at either end and pull the case with a quick jerk towards you. After you remove the top cover, with the front of the computer facing you, pull up on the plastic cover on the left side of the chassis. This plastic cover is on a hinge and will open towards the left. You will then see two plastic lock mechanisms on the side of the metal platform with the power supply and floppy drive -- they sort of look like a switch. Slide these locking mechanisms towards the center of the chassis and pull up on the platform. It is also on hinges and will open towards the right. At this point, you'll see the logic board and hard drive on the bottom of the case.
It uses an ATA/33 hard drive.