Thank you for the link.
Now I need to find out which file I need, as I do not know if the integrated ethernet/network hardware is Gigabyte, Broadcom, or whatever, or even if it's firmware I need.
Hardware Wizard says I have a device named 82557-based Integrated Ethernet, but the device cannot start. In any case, it is not a plug-in NIC.
PC is a second hand HP dc7700, if that is any help.
So far I have not been able to locate drivers or firmware that is related to whatever device I have.
Any further help is much appreciated.