Changing your startup disk in MacOS X never alters your other startup disks.
Since you're asking about the installer disc, I can only assume that you want to reinstall your OS, and are afraid of (or trying to) wipe the current disk clean to start over. You have to run the installer from the disc, then choose to either Erase and Install (a process that wipes your hard drive completely and installs a fresh, vanilla version of the OS - note all user accounts and files on that disk are gone too), Archive and Install (saves your current OS installation to the side in a new folder, and doesn't touch your user files, but then makes a fresh OS install), or Upgrade (which writes over the current OS, but only if the files on the install disc are newer).
Archive and Install is the safest, and the default.
Comments
Changing your startup disk in MacOS X never alters your other startup disks.
Since you're asking about the installer disc, I can only assume that you want to reinstall your OS, and are afraid of (or trying to) wipe the current disk clean to start over. You have to run the installer from the disc, then choose to either Erase and Install (a process that wipes your hard drive completely and installs a fresh, vanilla version of the OS - note all user accounts and files on that disk are gone too), Archive and Install (saves your current OS installation to the side in a new folder, and doesn't touch your user files, but then makes a fresh OS install), or Upgrade (which writes over the current OS, but only if the files on the install disc are newer).
Archive and Install is the safest, and the default.