A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

Facing issue when try to connect to MSSQL 2005 Express from remote server

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

To solve this issue

1. Enable TCP/IP


Go to Programs -> Microsoft SQL Server 2005 -> Configuration Tools -> SQL Server Configuration Manager

Make sure TCP/IP is Enabled

2. Make sure MS SQL allow remote connection with TCP/IP protocol.

Go to Programs -> Microsoft SQL Server 2005 -> Configuration Tools -> SQL Server Surface Area Configuration

Click on Surface Area Configuration for Services and Connections

Make sure Local and remote connections and using TCP/IP only is selected.

3. Open port for Port: 1433, sqlbrowser.exe and sqlservr.exe in windows firewall.

Launch the Windows Firewall configuration tool from the control panel.

Click the Exceptions Tab

Click on Add Port, key in information as below

Repeat above but this time click the "Add Programs." button and select "sqlservr.exe" from the 

location where you install SQL Server Express (normally in C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Binn)

Report the same steps but select "sqlbrowser.exe" (normally in: C:\Program Files\Microsoft SQL Server\90\Shared)

 Try again and the problem solved. Hope this help.