Your account rights must have changed, or someone changed rights on the drive to remove your account. Check with whoever is in charge of security for the resource you are working with.
Also, a network drive can never be C, you may want to explain things a bit better.
sorry for the late reply. I did not explain the problem very well as you may have guessed. First of all I am a network admin. If I logon to any PC on the network (remotely or locally), I can no longer add any users or change permissions to the C: drive. There are no problems with adding user's or permissions to any folder beneath the C: drive.
Also, there are no problems adding user's or permissions to the C: drive with a newly-cloned PC.
I cannot think what may have happened.