If you ever need to install (or reinstall) OS X on a drive other than your current boot drive, you can do it without rebooting. (Normally, when you launch the OS X installer, it reboots your machine before starting the installation.) With this hint, you can do the whole install process while booted normally from your current drive -- so you can keep working while the install is going on.
To do that, just launch the Unix executable for the installer application on the DVD, by running the following command in Terminal:
sudo /Volumes/Snow\ leopard\ Install\ DVD/System/Installation/CDIS/Mac\ OS\ X\ Installer.app/Contents/MacOS/Mac\ OS\ X\ Installer
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100122081044636