When a CMOS Checksum error occurs during system startup, it could mean that the system has been restored to default factory settings. The CMOS contains data related to BIOS and may have been reinitialized. It may also be possible to fix the problem by reinitialising BIOS. The problem could also occur due to the depletion of the CMOS battery. If the user cannot locate the pin battery, the help of a computer technician should be taken to prevent damage to the hardware or motherboard. The CMOS checksum error - defaults loaded should be solved after changing the CMOS battery.
If the problem persists this may be due to the fact that the battery powering the CMOS (when the computer is shut down it allows the CMOS to store information), is depleted.
The CMOS battery is a small pin found on the motherboard; you can refer to the manual provided with your motherboard to change it or have the nearby computer technician change it for you if you don't want to risk any damage.