You should only use a dedicated administrator connection (DAC) in special occasions. For example, in some cases, you may find that a user is running a query that has utilized all of your systems resources. The user may be using so much of the system’s resources that no other users, including yourself can login to the system to kill the users connection.
This would be a great use for the DAC. Under this situation before, you would have to stop and start the SQL Server if some time had passed. With the DAC enabled, you as a DBA can login if you had sysadmin rights by qualifying the server with ADMIN:. For example, if you wanted to login into the localhost machine you would use ADMIN:localhost. You will receive an error if the feature is disabled. To enable the feature, go to the SQL Server Surface Area Configuration. After it is enabed, you must stop and start the SQL Server instance.
For more information about Surface Area Configuration, go to http://www.jumpstarttv.com/Channel.aspx?cat=8a62d008-b580-4422-91f7-29515baedce3.
-- Brian Knight