SQL 2000 introduced multiple instances of the database. GP 9.0 has now introduced multiple instances of the program. After you've installed GP 9, you can actually do a second install that will be placed in a separate instance. This works well for testing since you install a complete test environment, DB and GP, on a machine that is already running Dynamics GP, without affecting the existing install. Right now for example, I have one environment with SP1 and one without. Both running on the same machine and pointed to different DB's.
Simply reinsert the Dynamics GP CD and click Install Microsoft Dynamics GP. In the course of the install, you will get a chance to name the instance. Pick a name and the whole install will be built around that name.