Login failed for user 'domain\username'. (Microsoft SQL Server, Error: 18456)

Microsoft SQL Server, Error: 18456 Solution by Pinal Dave:

1. Create a new user with Administrator privilege with the same username and password as SQL Server 2008 in the operating system.
   
2. On SQL Server database create a new user by expanding DatabaseNode >> Security >> Login >> Create New User and add this new user with the Windows Authentication radio button selected. This user can be only added by selected Windows Authentication it is the Operating system’s User Login. Once above. you should perform the above steps using windows authentication
   

Hope this helps you solve the Microsoft SQL server, error: 18456!