|>> I still have a belief that upgrading bios could solve this "CMOS check sum bad" problem. Any advise is appreciated. <<|
I am afraid you are working on a mistaken belief.
When you save the BIOS setting they are saved to CMOS memory. A checksum is calculated on the data and that is also saved to CMOS memorry.
When the computer boots again it calcuates the same checksum from the information stored in the CMOS memory. If the Chechsums match everything is fine. If the CMOS memory has been corrupted becuse there is no power to maintain it, the Checksum is incorrect and you get the error.
The CMOS checksum has nothing to do with the BIOS codes which is in ROM and retains its contents without power. It is virually impossible to corrupt BIOS memory unless you do something silly, like a bad flash.
When you checked the battery in the other computer did you remove all power to make sure that it was working and not just close down and re-boot. If that is all you did then the battery was not be put to the test.
If you are absolutley certain it is not the battery you have to be looking at some other reason they the CMOS memory is not retaining its settings. It may be that the CMOS memory itself is faulty in which case its a new motherboard or possibley a new BIOS chip as the CMOS memory is contained withing the BIOS chip.