"Not Associated with a trusted SQL Server Connection" when I try to connect to an SQL database and use a valid SQL username?

This is a very common question I have come across.
This occurs typically when Microsoft Sql Server installation is configured to use only Windows authentication and not SQL Server authentication.
So when u try to login using sql server account this error occurs.

 To solve the problem: Change the Authentication change the authentication mode to sql server authentication.
1. Start SQL Server Enterprise Manager (Start, Programs, Microsoft SQL Server, Enterprise Manager).
2. Navigate to the SQL Server system in the Microsoft Management Console (MMC) treeview pane by selecting Console Root,     
    Microsoft SQL Servers, SQL Server Group, (or ); right-click the server; and select Properties from the context menu.
3. In the SQL Server Properties dialog box, select the Security tab.
4. In the Security section, make sure that Authentication is set to "SQL Server and Windows" and not "Windows only" .