It is the "quarter size" battery. Still, a PC should be able to POST w/ a dying one, usually gives you a warning about replacing. Otherwise, it forgets your BIOS settings including date and time when your PC is shut down. I'd try removing and reinstalling battery. Unplug and plug in PSU. If no luck there, remove and reinstall RAM modules, power connectors, ect. How old is the system itself? Capacitors bulging at all around CPU socket?