About the Shut Down Problem:
If you don't disable your network connection, are you able to turn it off normally? I actually believe that they are unrelated.
The MCI error is being caused by a piece of a program that was not closed when the program was or is still processing information. This error may also be whats causing a normal shutdown to not work. What happens is that the normal shutdown can't override the MCI process to shut down the computer so it ends up giving up. Usually you would get an End Program error with a timer that will automatically end it. But certain programs can't be ended automatically so if it was one of those it's possible it could stop a normal shutdown.
You can try finding the MCI process, ending it, and then using a normal shutdown to see if that is whats causing all the shut down problems. To end it, press Ctrl + Alt + Del and go to the processes tabs. Click sort by User so that all the processes under your username are at the top. See if you have any processes with MCI in them, select it, and then press end process. DO NOT end any processes under SYSTEM LOCAL SERVICE or anything else except your User Name, also DO NOT end the explorer.exe process.
If you want to look for other solutions yourself there is plenty of information on it in google.
http://www.google.co...;fp=g9QnLpbgWtc
I did some looking myself but the information I found included no solutions.
As for your Network connection:
Are you using the Network Connections window to disable/enabled it, or a different method?
What is your purpose in disabling it and then re-enabling? (there may be a different way to do it)
Have you tried using a different cable? If not then see if you can find another cable to test it with.
Also are you connecting to a network to use the internet or to communicate with other computers on your network, or a different reason?