Bios error message!

Hello,
When I turn on my computer, before booting, I encounter an error message that says:

CMOS/GPNV checksum bad
Press <F4> to run setup

I haven't updated my bios and I don't remember changing any bios settings. I have also loaded optimal defaults in the bios setup but that also does not work. I also removed the CMOS battery and replaced after waiting for some time, clearing bios memory and loading the optimal defaults. But none of those ideas work. I don't see this error message when I restart my computer, but only when I turn off and turn on again ( even immediately).

This is not really a problem, because when I press F4 and in the bios screen if I press Esc without changing any settings, the computer boots normally. But I fear that this problem may render my computer unusuable if it continues to pop up.
Thank you.