I have a desktop computer that no longer boots. Sometimes I get an error message saying keyboard error, and it stops there. Other times, I just don't get any kind of error. I am unable to enter the BIOS setup, nor make it to the disc reading part of the boot process. Is this a motherboard issue?

Is this a motherboard issue?

Sounds like it, assuming the power supply is good. I would replace the battery - they are about $2 in the US and available at most battery/watch/camera counters. Unplug the computer from the wall, then touch bare metal of the case to discharge any static in your body before reaching in. If full of heat trapping dust, clean - but keep touching bare metal of the case. Take old battery with you as most counters recycle. Do not touch new battery with bare skin - I put a clean sock over my hand. At first boot, go straight into BIOS Setup Menu, set date and time, verify the drives are properly identified, then "Save" and Exit.

If that does not work, you are out only a couple bucks for the battery, and you probably need a new motherboard.