Hello.
This is because windows does not have the correct drivers for your ethernet adapter. You don't have the CD that comes with your motherboard? It has all the drivers you need for the ethernet. Just insert the CD, click setup/install and follow the instructions.
If you don't have a CD, I suggest downloading the drivers for the ethernet using another PC, then copying it on a USB flashdrive (or CD), and then installing it on your upgraded pc.
If you don't have access to another pc...a phone with a USB adapter can work.