I ran into this same issue. I removed avast from my system, however, I still continually get this message "An Ethernet cable is unplugged" and then "The system has been disconnectef from the media server". My router then blinks orange where usally it's green, then goes back to normal. I was able to access the media server (and thus my Windows 7 Home Premium PC) however, since the upgrade I cannot access. Even after uninstalling avast.
Any thoughts? Could it be my router?
I have already checked the ethernet cable and it is indeed plugged in (I can access PSN, and do online gaming so I'm still hardwired).