Follow any of the solutions below to resolve this issue:

Solution 1:

Open the SQL Connection Manager and if SQL TCP protocols are already set to Enabled, change the protocols to Disabled and then back to Enabled. Close and save the changes. Then restart the SQL Instance service.

 

Solution 2:

Modify the following registry key to specify a TCP port after reviewing the associated Microsoft Article:

support.microsoft.com/kb/823938

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.1\MSSQLServer\SuperSocketNetLib\AdminConnection\Tcp\TcpDynamicPorts

Solution 3:

Enable TCP Dynamic Ports if you have other instances of SQL Server 2005 installed on the same machine. Make sure that SQL Server [Bkupexec] is running before proceeding with the steps below.

To enable TCP Dynamic Ports:

a. Open SQL Server Configuration manager.
b. Expand Nework configuration on the left and click on Protocals for SQLEXPRESS

c. Right click and select properties on TCP/IP

d. Verify in the protocol tab Enabled is set to Yes

e. In IP Address tab set TCP Dynamic Ports value to 0 to turn on Dynamic port ranges.

3. Stop and restart the SQL Server [Bkupexec] service.

4. Stop and restart the SQLBrowser Service.
 

5. Make sure  there are no warnings received after  restarting SQL Server Express and SQLBrowser.