Well, got replacement board late this afternoon. With help of 4 yr old granddaughter, had the old one out and the new one installed in 45 minutes or so. Including a timeout to explain the difference between "plus" and "minus" screwdrivers.
Fired her up (PC, not granddaughter - she's fired up enough as it is), installed the latest LAN driver... and had the same exact problem!! Fortunately, granddaughter had joined Nana at the far end of the house by that time. Real bad feeling in the pit of my stomach too.
So.. uninstalled the LAN driver and reinstalled everything from the Gigabyte CD that accompanies the mobo. Viola! LAN connection works! I'm tempted to retry with the old mobo - but it's all packed up in its UPS box for return.
Unbelievable. Kicking myself for not trying that last time round.
Oh wait. Forgot to set ACHI in the new mobo before installing Windows. Grrr.
Change BIOS, reinstall Windows, reinstall drivers from Gigabyte CD.
No LAN connection. ARRRGH!!!!
And then, without me doing anything, after 10 minutes go by... the LAN is active. What next?!
Stay tuned.