If you cannot login to a remote servers database but the login is working from the localhost, and the port is open in Windows firewall, then you probably have to enable the remote connection in SQL Server Configuration Manager.

Since I found a great article about this subject I wont write it myself. A great guide is in the other end of this link http://www.linglom.com/2009/03/28/enable-remote-connection-on-sql-server-2008-express/