Hi.. I bought New PC a week ago.. i too find the same problem with Windows 8.1..........
the motherboard i have is intel DZ 75ML-45K.
Ethernet Connects perfectly in Win 7 & even in Linux.
I asked the store i bought the PC to rectify the problem.
They told they contacted Intel about it, and they find problem having suitable driver FOR windows 8.1
they told me, Perhaps it will take 2-3 months for new drivers to get released, and i would have to use wifi receiver or something else for it..
(the problem is with suitable Ethernet Drivers)
I know it is not the solution you would have been expecting, But just wanted to share what i got to know..
P.S. i would like to know what MotherBoard you have.. just for me to feel better..