I am wanting install database dotnetnuke on sql server 2005 and moment I give server name  in "mohsen" error down was displayed.

error:    login failed for user "NT AUTHORITY\NETWORK SERVICE"

why  occur this error?

It appears that you are trying to use Windows Authentication for the connection to your SQL Server.  Are you intending to do this, or would you prefer to use SQL Authentication (provide a username and password, instead of using the current Windows User account)?  Typically people use SQL authentication for ease of use.

Is this occuring in the install wizard, or are you setting the connection string in the web.config?

Hope it helps,

When you are using the Install Wizard, make sure that you have already created a database and a user who can access it.  Use that information in the Install Wizard.  Specifically, make sure that you uncheck the Integrated Security checkbox and provide the username and passward of the database user you created.

Let me know if that works for you,