Default is on. To disable the functionality of enabling user instances, clear the check box.
The user instance, also known as a child or client instance, is an instance of SQL Server that is generated by the parent instance (the primary instance that runs as a service, like SQLExpress) on behalf of a user. The user instance runs as a user process under the security context of that user. The user instance is isolated from the parent instance and any other user instances running on the computer. The user instance feature is also referred to as “Run As Normal User” (RANU).
Logins provisioned as members of the sysadmin fixed server role during setup are provisioned as administrators in the template database. They are members sysadmin fixed server role on the user instance unless removed
Add User to the SQL Server Administrator role
Default is not on. To add the current setup user to the SQL Server Administrator role, select the check box.
Windows Vista users that are members of BUILTIN\Administrators are not automatically added to the sysadmin fixed server role when they connect to SQL Server Express. Only Windows Vista users that have been explicitly added to a server-level administrator role can administer SQL Server Express. Any member of the Built-In\Users group can connect to the SQL Server Express instance, but they will have limited permissions to perform database tasks. For this reason, users whose SQL Server Express privileges are inherited from BUILTIN\Administrators and Built-In\Users in previous releases of Windows must be explicitly granted administrative privileges in instances of SQL Server Express running on Windows Vista.
To make any changes to the user roles after this installation program ends, use the SQL Server 2005 Surface Are Configuration Tool (SQLSAC.exe). To update the list of users in the SQL Server System Administrator role, click the Add New Administrator link.
Ensure that the User to provision field lists the DomainName\UserName of the user whose permissions should be updated. Select the role to be updated from the list of SQL Server instances in the Available privileges pane, and then click the right arrow. To add the user to all available roles for all available instances of SQL Server instances and all available roles, click the double right arrow.
To implement the changes when your selections are complete, click OK. To end the tool without making changes, click Cancel.