Drivers for the Ethernet adaptor : Did Windows find these for you and install, or did you download from the Atheros site?
This seems to be the download page.
http://www.atheros.com/corporate/content.php?nav1=146Under "AR813X AR815X Family Drivers" choose "WIN_AR813X_AR815X_Windows WHQL Driver V2.1.0.5"
I'm still thinking here that there's something up with the install of the network card, but, as has been suggested it could also be the switch. I assume you've not got another gigabit device to try?
Here's what I'd do next :
Download that driver, save it onto the errant machine, ready to install.
Shut down, remove the network card from the computer, boot it up as normal again.
Find Command Prompt in start menu, right click on it and "run as administrator"
type the following :
set devmgr_show_nonpresent_devices=1
start devmgmt.msc
Device manager now starts.
Click "view" and tick "show hidden devices"
You can now see all devices that have ever been in the computer. Devices that are no longer there appear in a light grey.
Under network adaptors, find the ethernet controller, highlight it and click the "x uninstall" button at the top. If an option appears asking if you want to remove the driver as well, then do so.
Shut down, replace the network card, and reboot. When the card is detected, point it to your newly downloaded driver.
I always prefer a fresh install of a driver to an upgrade
Ian