user instances enabled Option
The user instance enabled option that you can access through sp_configure is not supported in Microsoft SQL Server 2008. This option works only with SQL Server 2008 Express (SQL Server Express).
This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.
For more information about how user instances work, see User Instances for Non-Administrators.