SQL Server logon through Windows authentication checks whether a trusted connection exists (i.e. successful logon to Windows NT or 98/95). The properties of a trusted connection are retrieved from the Windows NT logon. If a trusted connection exists, the SQL Server will allow access wihtout requiring additional logins.

If you try to connect to an SQL Server instance with a blank user name, the SQL Server automatically uses the Windows NT account. If you try to connect to an SQL Server instance with an SQL account, but the SQL Server instance is configured for Windows logon, the SQL logon will be ignored and the Windows logon used instead.

Windows authentication has apparent advantages over SQL authentication: integration with Windows security policy, centraliation, secure authentication, password encryption, access control, minimum password length, etc.

Windows authentication is not possible if the SQL Server instance runs under Windows 9x.

Enabling Windows Authentication

In SQL Server Enterprise Manager

1. Open the SQL Server Enterprice Manager (usually Start | All Programs | Microsoft SQL Server | Enterprise Manager).
2. Find the desired SQL Server instance, right-click on it, and select Properties.
3. Go to the Security tab.
4. Select the option Windows only.
5. To enable or disable login attempts, select one of the options under Audit level:  None to disable it, Failure to log only failed login attempts, Success to log only successful login attempts, or All to log all login attempts.