This is how to get ‘back into’ a SQL instance when the local administrators group have been ‘locked out’ by not being SYSADMIN on the sql instance (and the SA password has been lost / other admin accounts are unknown / inaccessible)
On more than one occasion people who should know better have flat-out told me that this can’t be done, so just while I have the link handy:
…if SQL Server 2005 is started in single-user mode, any user who has membership in the BUILTIN\Administrators group can connect to SQL Server 2005 as a SQL Server administrator. The user can connect regardless of whether the BUILTIN\Administrators group has been granted a server login that is provisioned in the SYSADMIN fixed server role. This behavior is by design. This behavior is intended to be used for data recovery scenarios.
This is also true for Sql 2008. See Starting SQL Server In Single-User Mode