It is probably the battery the supplies power to the little CMOS RAM module that is dead/flat.
Normally, when you go into the BIOS setup and adjust the time, date, boot sequence and other stuff, the settings are stored in the CMOS RAM module with a checksum value. When the PC is booted after a power-down, the BIOS code will read the values of the CMOS RAM, recalculate the checksum and compare it with the old value. But if the battery is dead, the values will be garbage, and the checksum will be garbage - and you get that Checksum Error message.
Replacing the battery can be easy - or it can be almost impossible. It depends on the type of battery used. Some are soldered into the motherboard and almost impossible to replace. Others are like simple watch batteries, a shiny coin-looking thingy that you can buy in any supermarket, and quickly replace.
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