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The SQL Server Agent Service does not start and you may see the following error message in the Windows Event Viewer:
SQLServerAgent could not be started (reason: Unable to connect to server; SQLServerAgent cannot start).
Additionally, if you attempt to start the SQL Server Agent Services from a command line (for example, C:\MSSQL7\BINN\>sqlagent -c -v) you may see the following error messages in the C:\MSSQL7\LOG\SQLAGENT.OUT file:
7/13/99 2:07:43 PM - ?  SQLServerAgent starting under Windows NT service control 7/13/99 2:07:43 PM - !  SQLServer Error: 18456, Login failed for user 'DomainName\UserName'. [SQLSTATE 28000] 7/13/99 2:07:43 PM - !  Unable to connect to server; SQLServerAgent cannot start 7/13/99 2:07:44 PM - ?  SQLServerAgent terminated (normally)
The BUILTIN\Administrators login does not have access to SQL Server or the BUILTIN\Administrators login has been removed from SQL Server, and the SQLAgent service is starting under the LocalSystem account.
The issue does not occur if the SQLAgent service is starting under a domain or computer account that is listed separately in SQL Server Enterprise Manager and that has System Administrators (sysadmin) privileges.
To put the BUILTIN\Administrators account back into SQL Server, start the SQL Enterprise Manager. Go to \Servername\Security\Logins and grant access to the BUILTIN\Administrators user.
If you really want to deny access to SQL Server by Windows NT administrators, you can remove the BUILTIN\Administrators account. First add the login account you are using for the SQL Agent Service as a SQL Server login and assign the System Administrator (sa) role to that account. Then either revoke the System Administrators (sysadmin) role permissions from the BUILTIN\Administrators account, or delete the BUILTIN\Administrators account completely from the Logins in SQL Server Enterprise Manager.
For additional information, see the SQL Server Books Online topic "Viewing and Modifying Role Memberships," which is available at the following MSDN Web site:
Note that when you run the service by clicking Control Panel and then choosing Services that there are not any helpful error messages. You have to check the Windows Event Viewer or start the service from the command prompt to see them.