Cmos checksum error - defaults loaded

I am working on an older Windows 98 computer. When I turn it on, the screen shows the bios manufacturer and version number, identifies the primary and secondary drives, then posts the following error message:

CMOS Checksum Error - Defaults Loaded

Press F1 to continue, DEL to enter setup.

Pressing either key does not work, as a matter of fact the lights on the keyboard flicker, but none of the keys will work. tried pressing Cntrl-Alt-Del to reboot but nothing happens. I replaced the cmos battery with one that I know is good and get the same results. I cannot enter setup or the boot menu to diagnose the problem. I think there is something wrong with the PS2 port that the keyboard is plugged into, because I know the keyboard works in another computer.

There is only one PS2 port, the other port is a larger 5 pin port, normally used for a mouse. Could this 5 pin port also be used for a keyboard, and maybe I need a keyboard with a 5 pin plug to make the computer boot?

Or . . . am I diagnosing the problem incorrectly?