Have you ever needed to reinstall or upgrade Mac OS X without worrying about copying over your existing user data to an external hard drive? It's possible in Mac OS X to reinstall or upgrade without losing any other existing user data on the system. Once you've installed the fresh operating system, you can transfer your documents, movies and music on your own time without worrying about external hard drives.
Insert the Mac OS X installation disc and restart the computer.
Hold down the "C" key while the computer is restarting until the Mac OS X installation utility appears.
Click a language from the list, then click "Continue."
Click "Continue" on the "Welcome" screen.
Click the drive you want the operating system installed to from the "Select a Destination" screen, then click "Options."
Click the "Archive and Install" option, check the "Preserve Users and Network Settings" option, and click "OK."
Click "Continue" on the "Select a Destination" screen, then click "Install" on the "Install Summary" screen to begin the installation process.
The "Archive and Install" option creates a new partition on your system hard drive to install a fresh instance of the operating system, but leaves your user data on the old partition. It's important to check the "Preserve Users and Network Settings" option so that all of your user profiles and saved information (such as wireless networks) is automatically moved over to the fresh operating system. One reason to uncheck the "Preserve Users and Network Settings" option is if you suspected malware on your system, or if you no longer the user profiles and network settings.
Make sure you back up your hard drive before installing the new operating system in case of hard disk failure.