This error crops up quite frequently together with "Not associated with a trusted SQL Server connection" error message when you try to connect to SQL Server 2005.
Ensure that all the domain controllers are correctly processing domain logins. Problems in AD may cause this problem if the SQL server is unable to authenticate the login.
Set SQL server authentication to mixed mode and restart the service if needed.
On the SQL server, check whether the Windows login/group which is being used is found under the Local policies | User rights assignment | Access this computer from the network