Кажется, нужно зарегестрировать одну dll-ку в Extended Stored Procedures на сервере.
В 4 - ке так, не знаю как в 2.6, может быть также.
Extended Stored Procedure
-------------------------
With the Microsoft SQL Server Option for Microsoft Business Solutions-Navision,
you can use Windows authentication to connect to a
Microsoft SQL Server. But this is only possible when the
server's operating system is Windows XP, Windows 2000 or Windows NT 4.0.
However, in order to use Windows authentication, you must
add an extended stored procedure to each SQL Server you
want to access.