Remove the medium-length screw securing the optical drive, and slide the optical drive out.

Remove the medium screw securing the motherboard to the base.

Remove the short screw securing the modem card to the base.

Lastly, remove the two VGA screws.

Flip the motherboard over, and there's the BIOS battery.

The battery is an ML1220 3-volt rechargeable cell.

Do NOT replace the cell with any other type of cell or battery.