If you have encountered this error and would like to fix your computer, here are some remedies for each situation:
CMOS Battery Not Functioning Properly: If you suspect your CMOS battery is not functioning properly you can easily change it. Before changing your battery, reboot your computer to make sure that the error still exists. If it does, go into your CMOS and write down all of the settings. If all settings are lost, you can usually get them from your computer manufacturer. Now locate the battery and remove it, you might need to consult your computers manual or tech support to remove your battery. Take down the CMOS batteries information such as volt, size, etc. Once you have your new battery, you can replace it and reenter your CMOS settings. If your battery was the cause of the CMOS Checksum Bad Error, you should have remedied the problem.
Your BIOS has Been Updated (either by you or possibly a virus): If your BIOS has been updated recently, your CMOS settings may have become reset. Make sure that the values entered in the BIOS are correct or simply reset them to the default settings. If you believe that a virus updated your BIOS settings, run a virus scan and make sure that the BIOS settings are back to the default.
The Computer Was Shut Down Improperly: Sometimes when running MS Windows, if you shut down your laptop or desktop without first properly shutting down your operating system it will corrupt the CMOS settings causing the CMOS Checksum Bad Error. You can easily avoid this error by making sure that you completely shut down your computer before turning off the power. Usually this entails going into the Start Menu/ Turn Off Computer/ Shut Down. If you received the error, shut down the computer properly, if this is the cause of the error, the error message will not return.