Here is what you want to do (this goes only if your motherboard is Gigabyte
K8NSC-939):
1. Download
latest BIOS
2. Unpack it on bootable floppy disk (it will overwrite autoexec.bat)
3. Delete config.sys on the floppy
4. Boot the machine from floppy and let it flash your BIOS. It takes a minute or 2. Do not interrupt the flashing process. If you do you will render your machine useless. Recoverable, but not necessary.
If you can't set your BIOS to boot from floppy due to inability to save settings, just unplug HD and CD drive, leaving your system no options but to boot from floppy.
This will make sure your BIOS is functional and not corrupted, like it seems.
If the CMOS errors appears again, it is likely that the CMOS battery is to blame. Replacing it would be my advice then.
Also, some modems are known to cause some weird boot behavior. If you are using one, try running machine without it.