Use the Installation Type page of the SQL Server Installation Wizard to specify whether to install a new instance of SQL Server 2008, or add features to an existing instance.
Select the radio button that specifies your choice:
Perform a new installation of SQL Server 2008
Add features to an existing instance of SQL Server 2008
If you select the option to add features to an existing instance, use the drop-down list to select the instance of SQL Server to update.
You cannot add features to a failover cluster instance after it has been installed. To add SQL Server features to an existing failover cluster, you must perform a new installation to install a separate instance of SQL Server.