This is a known bug in the nVidea drivers. Until the problem is fixed by nVidea I just leave my system running 24/7. They are promising a fix in the next few days if you can believe that.
I do not find it necessary to unplug after shutdown. When it is initially restarted after a power off it will hang mid way through windows load. Another restart and it will load windows normally.
One of the prices of being an early adopter