You can enter into single user mode for a database or for the entire SQL instance. To change a Single SQL Database to single user use the following SQL command (replacing DBNAME with your desired database).
EXEC sp_dboption 'DBNAME'', 'single user', 'true'
To change back to Multi-User use the following.
EXEC sp_dboption 'DBNAME', 'single user', 'false'
To move your SQL server into Single User Mode (used mainly to restore System Databases). You have to start the SQL Server Service with an start parameter of "-s -m" if it is the Default Instance or "-s{ Instance name } -m " if it is a named instance. You can get more information here:
http://www.databk.com/start-sql-single-mode.htm
