CMOS Battery
On many desktop computer motherboards the CMOS battery is a simple CR2032 Lithium coin cell which is easy and cheap to replace. The typical life of these cells is around 5 years.
Picture of a CMOS CR2032 battery, the BIOS EEPROM chip can be seen on the left
If the CMOS battery fails or is failing there are several symptoms: The computer's real time clock might be running slow; the BIOS setting might keep reverting to their default state (the CMOS battery provides power to the CMOS memory holding the BIOS settings); a 'Low Battery' or 'CMOS checksum error' warning might appear on the computer screen; the computer might refuse to boot.
Preventive maintenance
If the CMOS battery is a simple coin cell type it's a good idea to replace it every three or four years, but before you do make a note of the BIOS settings as these will be lost when the old battery is removed, and will have to be re-entered when the new battery is installed.