Hi Nadya,
Can you log in a the System Administrator, (e.g. sa). If you can, then you should be able to change the username/password pair assigned to Northwind Table through the MSSQL database administration GUI.
Since your building an app for the web, passwords and logins will probably not be an issue. When you access the MSSQL via the web, it is being accessed locally as localhost rather than remotely; because, it is really being called by the web server via an asp script. You might consider setting up Trusted Authentication during application development. When the app is ready, you can enter the host as localhost and the username/password pair need to access the apps tables. Later you could delve more deeply into MSSQL authentication/security.
LelandJ