I have lost the ADMIN$ share.

A. A. If you have configured the system to not automatically create system shares at start-up time by setting the relevant registry entry AutoSharexxx then this share will not be created as that is what you are asking.

If however you do not have this set and you have just lost the ADMIN$ share which points to the %SytemRoot% folder, e.g. d:winnt then you can recreate it by entering the following command:

C:> net share admin$

For more information on suppressing the system shares please see Q. How do I stop the default admin shares from being created?