SQL Server 2005 and 2000 on same machine

The installation of SQL Server 2005 and 2000 on the same machine may lead to the problem that the SQL Server 2000 server will not be visible to the client machines. This problem can be resolved using the SQL Server Browser Service which comes along with the SQL Server 2005 installation

What is Browser Service?

SQL Server Browser runs as a Windows service on the server. SQL Server Browser listens for incoming requests for SQL Server resources and provides information about SQL Server instances that are installed on the computer. SQL Server Browser contributes to three actions:

• Browsing a list of available servers
• Connecting to the correct server instance
• Connecting to Dedicated Administrator Connection (DAC) endpoints

For each instance of the Database Engine, the SQL Server Browser service (sqlbrowser) provides the instance name and the version number. SQL Server Browser is installed with SQL Server 2005 and provides assistance for previous versions of SQL Server that are running on that computer, starting with SQL Server 7.0.
In SQL Server 2000, the identification of the server connection endpoints was performed by the SQL Server service. SQL Server 2005 replaces that function with the SQL Server Browser service. If you install SQL Server on a computer that is also running SQL Server 2000 or MSDE, they must be upgraded to SP3 or later. Versions earlier than SP3 do not properly share port 1434 and might not make your SQL Server instances available to requesting client applications. Although you can change the startup order so that the SQL Server Browser service starts before SQL Server 2000 or MSDE, the recommended resolution is to update all older versions of SQL Server to the latest service pack.

Trouble shooting using Browser Service

When an instance of SQL Server 2000 is installed on the computer, if SQL Server Browser is not running, the SQL Server 2000 listener service will start. If SQL Server Browser starts after the listener service, it waits five seconds for SQL Server 2000 to give up port 1434. If that does not occur, SQL Server Browser will not start.
To resolve this problem with versions of SQL Server 2000 earlier than SP3, stop SQL Server 2000, start SQL Server Browser, and then restart SQL Server 2000. The SQL Server 2000 listener service will continue to try to start on port 1434, so the SQL Server 2000 instance should be upgraded to SP3 as soon as possible.
when your application is accessing SQL Server across a network, if you stop or disable the SQL Server Browser service, you must assign a specific port number to each instance and write your client application code to always use that port number.