From time to time it may be necessary to reinstall OS X to correct errors and improve computer performance. The reinstallation process takes only a few minutes and is practically automatic. As long as you have a good backup of all your important files, you will also avoid a lot of headaches. Go to the first step to learn how to reinstall OS X.
Steps
Part 1 of 3: Prepare for Installation

Step 1. Make a backup of the data
Reinstalling OS X will remove all data from the computer. For this reason, you must ensure that all important documents, images, videos and other files are copied to at least one other storage device.
- You can burn the files to a DVD, copy them to an external hard drive, or even upload them to a cloud storage service.
- Make sure everything you need is stored correctly. Once you install, you will not be able to recover the files.
- You can choose to export all settings and files during the installation process, but it is recommended to do a full installation where everything is removed for best performance.

Step 2. Begin the installation from an active computer
If the computer can boot OS X, you can start the installation process within the operating system. Insert the installation DVD into your computer and wait for it to appear on your desktop. Double-click the "Install Mac OS X" icon and click the "Restart" button.

Step 3. Start the installation from an idle computer
If your computer won't boot OS X, you can start the installation by booting it from the DVD. Restart the computer with the "Option" key pressed. This will load the "Boot Manager", which shows all the sources from which you can boot the computer.
Once you are at the "Boot Manager" screen, insert the OS X installation DVD. After a few moments, the DVD will appear in the source list. Select boot computer from DVD
Part 2 of 3: Install OS X

Step 1. Choose the language and start the installation
After the computer restarts, the installation process begins. You will be asked to choose a language and then the "Welcome" screen will appear. Click the "Continue" button to begin the installation.

Step 2. Choose the type of installation
Click the "Options …" button on the "Select a destination" screen. When you reinstall OS X you have two different installation options: "Save and install" and "Erase and install". Choose the process that suits your needs and click the "OK" button.
- "Save and Install" creates a copy of the files on the system and then installs a new copy. If you choose this option, you can also choose to keep your user and connection settings. This is not recommended if you have problems with the current installation of OS X. Any program that you have you will have to reinstall when choosing this method, otherwise it will not work properly.
- "Erase and Install" removes everything on the disk and installs a fresh copy of OS X. All data will be removed, so make sure you have a backup of everything that is important. This is the most recommended option as it solves most of the problems you may experience and offers the best performance.

Step 3. Choose the destination
If you have multiple hard drives or partitions installed on your computer, you can choose which one you want to install OS X on. The amount of disk space will be displayed and the amount required for OS X. Select the hard drive where you want to install it and click "Continue".
- Use the "Format disk as" drop-down menu to set the disk format to "Mac OS X Extended (Journaled)."
- Make sure not to install it on a recovery or storage drive.

Step 4. Choose the programs you want to install
The installer will list all the additional program packages that will be installed with OS X. If you don't have much space on your hard drive, you can remove some of the less important files by clicking the "Custom" button.
- Expand the "Printer Drivers" section and uncheck any of the printer drivers you don't need.
- Expand the "Language Translation" section and uncheck the languages you are not going to use.

Step 5. Start the installation
Once you have selected the additional program options, you can begin the installation of the operating system. Click the "Install" button to begin.
A progress bar will tell you how much time is left to finish the installation. The installation process is automatic once it begins. The computer will restart when complete
Part 3 of 3: Configure OS X

Step 1. Configure the keyboard
The first thing you will have to do once the computer restarts is to configure the keyboard. Follow the onscreen instructions to detect and configure the keyboard.

Step 2. Choose your region and keyboard layout
Once the keyboard has been detected, you will need to set the region and choose the keyboard layout. If you travel often and take your computer with you, choose the region in which you live.

Step 3. Choose whether or not you want to transfer data
When you install from scratch there will be no data to import. You will be able to copy the backup data later. Choose "Do not transfer my information now" and click "Continue".

Step 4. Enter the Apple ID
If you have an Apple ID, you can use it to sign in. This will sync the settings with all the Apple devices you have. Take a look at this guide to learn more about how to create an Apple ID. You do not need to enter an Apple ID.
You can also choose whether or not you want to register the program with Apple. This can help you if you need to get official support

Step 5. Create an administrator account
The administrator account is one that has permission to change system settings and install programs. If the computer is yours, enter your name in the "Name" field and a username in the "Short name" field. Sometimes users use a diminutive of their name in this last field.
- The short name is used to label the directory "Home".
- It is very difficult to change this name afterwards, so make sure you agree with it.
- The administrator account requires a password. You can also add a hint to remember it if you wish.

Step 6. Start using OS X
Once the "Setup Wizard" has finished, you can start using the newly installed operating system. You will need to reinstall whatever program you had before and you will be able to copy the backup files to the "User" folders.

Step 7. Install the available updates
After installing OS X, install all available updates as soon as possible. These help keep the system secure and improve performance. You will need to download them from Apple, so the computer will have to be connected to the Internet.
- To get the latest updates, click on the "Apple" menu and select "Software update …". This tool will check for updates and display them. Select the ones you want to install and click the "Install" button. The updates will be downloaded from Apple's servers and installed. Once the installation is finished you will have to restart the computer.
- Repeat the process. Some updates are available after previously installing others. Check and install all updates until there are none.
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Advice
Always complete any official Apple Program Update after reinstalling the operating system
Warnings
- Before carrying out any type of reinstallation, always make a backup copy of your files. Even when you do the Save and Install option, errors during the installation process can cause data to be lost.
- When performing the Save and Install option, use the installation CD for the version of the operating system you have installed on your computer. For example, if your computer was shipped with Mac OS X 10.4 (Tiger) but you didn't upgrade to Mac OS X 10.5 (Leopard), perform Save and Install using the Leopard CD.