Beginning in SQL Server 2005, significant changes were implemented to make sure that SQL Server is more secure than earlier versions. Changes included a "secure by design, secure by default, and secure in deployment" strategy designed to help protect the server instance and its databases from security attacks.
This strategy is further enhanced in SQL Server 2008 and SQL Server 2008 R2. These enhancements further decrease the surface and attack area for SQL Server and its databases by instituting a policy of "Least Privileged" and increase the separation between the Windows Administrators and the SQL Server administrators.
In SQL Server 2008 and later, the local Windows Group BUILTIN\Administrator is no longer provisioned as a login in the SQL Server sysadmin fixed server role by default at SQL Server setup install. As a result, box administrators cannot login to the new SQL Server 2008 and SQL Server 2008 R2 instance by default.
With SQL Server 2008 and later, during new installation, one or more windows principals are required to be provisioned in the SQL Server sysadmin fixed server role. Carefully provision the logins in the sysadmin server role membership as this is a very privilege server role.