The complementary metal-oxide semiconductor (CMOS) battery is a tiny watch-sized battery in your computer motherboard. The function of the CMOS battery is to record and save information for the system BIOS such as pre-boot user settings, your BIOS password and the computer clock. For one reason or another, the CMOS battery on your Compaq Presario 1200 may stop working; one sign of a dead CMOS battery is if the Windows clock is constantly reset when you power the unit on.
Disconnect the charger cable from your laptop. Slide the cover off the battery compartment on the right side of the computer. Remove the battery from the laptop.
Turn the laptop over and remove the four screws on the front-bottom side of the Compaq Presario 1200. Turn the computer back over and open the display screen.
Separate the palmrest from the laptop case at the front seam. Open the palmrest completely (there is a hinge near the keyboard) and gain access to the internal touchpad connector. Disconnect the connector by pulling the wide green cable out of the slot.
Remove the palmrest from the laptop by snapping it off the hinges. Separate the keyboard from the case by folding it over onto the compartment where the palmrest was. Be careful not to damage the connecting keyboard cable.
Unscrew the two screws in the heatspreader plate under the keyboard. Lift the heatspreader directly up to remove it from the laptop.
Unscrew the two screws in the modem, located on the left of the unit, under the heatspreader plate. Unplug the connecting cable and remove the modem from the laptop.
Locate the CMOS battery in the lower left corner of the motherboard. Remove the battery using your fingers to pry it free. Make note of the orientation of the old battery.
Replace the battery with a CMOS battery of equal size and voltage. Ensure that you install the battery properly, following the same orientation as the old battery.
Reassemble the laptop following the disassembly instructions in reverse. Power on your computer and verify that the new CMOS battery works by setting the clock and then rebooting the computer.