How can I determine installed SQL Server instances and their versions?

At a command line:

SQLCMD -L

or

OSQL -L

(Note: must be a capital L)

This will list all the sql servers installed on your network. There are configuration options you can set to prevent a SQL Server from showing in the list. To do this...

At command line:

svrnetcn

In the enabled protocols list, select 'TCP/IP', then click properties. There is a check box for 'Hide server'.

You could query this registry value to get the SQL version directly:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\90\Tools\ClientSetup\CurrentVersion

Alternatively you can query your instance name and then use sqlcmd with your instance name that you would like:

To see your instance name:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names

Then execute this:

SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition')

If you are using C++ you can use this code to get the registry information.