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Reinstalling OS X will erase all of the data on your computer. Because of this, you should ensure that all of your important documents, photos, videos, and other files are copied onto at least one other storage location.  You can burn your backup files to a DVD, copy them to an external hard drive, or even upload them to online storage. Make sure that everything you need is safely backed up. Once you install, you won't be able to retrieve the files anymore. You can choose to export all of your user settings and files during the installation process, but performing a clean install where everything is deleted is recommended for the best performance. If your computer can boot into OS X, you can start the installation process from 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 then click Restart. If your computer is not booting into OS X, you can start the installation by booting from the DVD. Reboot the computer while holding the ⌥ Option key. This will load the "Startup Manager", which displays all of the sources you can boot from. Once you are on the Startup Manager screen, insert the OS X installation DVD. After a few moments, the DVD will appear on the list of sources. Select it to reboot the computer and boot from the DVD.
Backup your data. Start the installation from a working computer. Start the installation from a nonworking computer.