Reinstall Your Mac OS X

Although rare, there are times when it is necessary to reinstall your Mac OS X operating system. With all of the different installation options this process can seem overwhelming and complicated; this article should help you make sense of the entire installation process.

Note: The steps in this article apply for version of Mac OS X up to and including Leopard. Snow Leopard has a similar process but will "Archive and Install" by default. To preform an "Erase and Install" on Snow Leopard you must erase the drive using Disk Utility before running through the installer.

Please Follow these steps carefully to prevent loss of data if you are reinstalling.

Back Up Before You Reinstall

Before performing any type of reinstallation, be sure to back up your personal files. Even when using the Archive and Install feature there is always a chance of data loss.

1. Decide on a backup location.

• External Hard Drive: this is the preferred method as a hard drive is capable of holding large amounts of data.
• CDs/DVDs: this is only an option if the computer is currently bootable and has a disc burner. CDs and DVDs are limited in their capacity, however, they are readily available.
• On-line Storage: if you have an FTP server or MobileMe account you may be able to back up your files on-line.

2. Decide what to backup.

• Personal Files: your personal files are located in your home folder. These include documents, pictures, movies, etc..
• Applications: it is generally not recommended that you back up applications, as people have a tendency to back up all of their applications including ones they no longer use. The reinstallation may be an opportunity to get rid of some of the unused applications, as you can easily reinstall them as they are needed.
• System Files: system files should not be backed up as they will be reinstalled during this process.

3. Enable the Root user on your computer. 

• Mac OS X 10.6 Snow Leopard
• Open Directory Utility in /System/Library/CoreServices.
• Click the padlock icon and enter your Administrator login.
• Select the "Edit" menu, and choose Enable Root User.
• Choose a password for the root user and click "OK".
• Mac OS X 10.5 Leopard
• From the Finder's Go menu, select Utilities.
• Open the Directory Utility.
• Enter your administrator password and click "OK".
• Select the "Edit" menu, and choose Enable Root User.
• Choose a Root user password and click "OK".
• Mac OS X 10.4 Tiger and Earlier Versions
• From the Finder's Go menu, select Utilities.
• Open the NetInfo Manager Utility.
• Find and click on the lock icon on the Manager window.
• Enter an administrator password and click "OK".
• Select the Security menu, and choose Enable Root User.
• Choose a Root user password and click "OK".

4. Log out of your current account.  

• Click on the Apple icon in the top left corner of the screen.
• Select "Log off <account name>".

5. Log in as the Root user.

• On the log in screen click "Other".
• Enter "Root" as the new name.
• Enter the password you created.

6. Using the Root user, drag the files and folders you decided to preserve to your backup location. 

• Personal files can be located by opening "Macintosh HD" and clicking the "Users" folder.

Select an Install Type

Mac OS X has three main reinstall types. Before you proceed it is important that you understand the differences between each. Choose carefully which type you would like to perform.

Archive and Install

• Available in Mac OS X 10.2 and later, an Archive and Install will automatically move existing system components to a folder called "Previous System". The Archive and Install will then proceed to reinstall Mac OS X.
• The Archive and Install will also preserve user account information and personal files. however, before proceeding with usage of the computer all software updates will need to be completed unless you are on Snow Leopard in which case the updates are preserved.

Archive and Install (Not Preserving User & Network Settings)

• Similar to the Archive and Install method, this option will move the existing system components to a "Previous System" folder; however, it also moves the user accounts and network settings to this location.
• If you follow this type of installation, you will need to recreate your accounts and reconfigure your network settings; however, your personal files are preserved on the hard drive and can be retrieved.

Erase and Install 

• The Erase and Install - as its name implies - will completely erase the contents of the hard drive before reinstalling Mac OS X.

Reinstall Mac OS X

1. Insert your Mac OS X Install DVD or Installation Disc 1.
2. Restart the computer and hold down the letter "C" as the computer starts up.
3. Continue to hold down the letter "C" until you see a grey screen with an Apple and a small spinning gear.
4. Select your language on the Welcome to the Mac OS X Installer screen and click "Continue".
5. Click "Continue" on the following screen.
6. After reading the Software License Agreement, click "Agree".
7. Select the volume to which you are going to install. "Macintosh HD" is the default name.
8. Click the "Options" button in the bottom corner of the window and select your installation type.
9. Click "OK" and then "Continue". You will be presented with a confirmation window at which point you can click "Install" to proceed.

After the Installation

After any type of installation there are a number of steps you should take to bring the computer back to a usable state.

1. Connect to the internet and run Software Updates. 

1. Click the Apple icon in the top left of the screen.
2. Select "Software Update"
3. Install all available updates.

2. Run a quick Repair Disk Permissions.

1. From the Finder's Go menu, select Utilities.
2. Open Disk Utility.
3. Select "Macintosh HD" on the left hand side of the window.
4. Click "Repair Disk Permissions" and allow the scan to complete.

3. Restore any files that you backed up to the location of your choosing.
4. Reinstall any software affected by the reinstall.

Things You'll Need

• A location to backup important files.
• Your Mac OS X Install & Restore CD/DVD or a retail OS X Installation Disc.

Tips

• Although the Archive and Install is the preferred method, some third-party applications, such as Microsoft Office, may need to be reinstalled as a result.
• Always complete any official Apple Software Updates after reinstalling the operating system.

Warnings

• Before performing any type of reinstallation, always back up your files. Even when performing an Archive and Install, errors during the installation process could result in a loss of data.
• When performing an Archive and Install, use the installation CD for the current version of the operating system that installed on the computer. For example: If your computer came with Mac OS X 10.4 (Tiger) but you upgraded to Mac OS X 10.5 (Leopard), Archive and Install using the Leopard CD's.

 source : WikiHow