Check http://blogs.msdn.com/sql_protocols/archive/2008/05/03/understanding-the-error-message-login-failed-for-user-the-user-is-not-associated-with-a-trusted-sql-server-connection.aspx

>Windows Authentication
>
>
>>Are you using SQL Server or Windows authentication in your application?
>>
>>>We have been having this problem the last few days where every so often when we run our app
>>>or even try to connection we get:
>>>
>>>

>>>Login failed for user ''. The user is not associated with a trusted SQL Server connection.
>>> (Microsoft SQL Server, Error: 18452)
>>>

>>>
>>>No changes have been made and the server machine is running normally.
>>>
>>>Anyone?