The BIOS battery in a Compaq Nc6000 laptop saves your date, time and BIOS settings, along with storing a BIOS password that you have to input in order to unlock the system at start-up. If you've noticed that your date and time settings are not being saved when you shut down, or that your password screen is either not present or asks you for a new password, you should replace the BIOS battery.
Switch off your laptop, unplug it and flip it over. Remove the laptop's main battery to avoid any electrical shock or damage. Unscrew the only compartment directly in the middle of the laptop and remove it (this is a RAM compartment, too).
Locate the BIOS battery. It is a thumb-sized, circular, black battery connected to the computer with a small wire harness. Unplug this harness and carefully pull the battery out.
Plug the new BIOS battery into the same spot where you removed the old one. Be sure that the wire harness clicks into place and that the connection feels secure. Replace the compartment cover, screws and main battery.