Its not a question of which one it defaults to. When installing the second one, you probably installed it as a Named Instance. If you didn't install one has named instance, you probably upgraded the 2000 to 2005. To access the named instance it would be machinename\instancename. If you just connect to the server as the machinename you are connecting to the main instance, which is probably the first one you installed (my guess 2000). One thing you can do in a query window to check is do a SELECT @@VERSION.