A clean install is an operating system (OS) installation that overwrites all other content on the hard disk. Unlike a typical OS upgrade, a clean install removes the current operating system and user files during the installation process. When a clean install finishes, the hard disk only contains the new operating system, similar to a computer that is used for the first time.
In most cases, a clean install is not necessary when upgrading your operating system. It is much easier and safer to perform a standard "upgrade and install," which simply upgrades the necessary files and leaves the user files in place. However, sometimes an OS upgrade is not possible because important files have become lost or corrupted. In this case, a clean install may be the only option. Some users may also prefer to perform a clean install so that no lingering problems from the previous OS will affect the newly installed operating system. Additionally, a clean install may be appropriate when installing an OS on a new hard drive or when transferring ownership of a computer to another person.
Both Windows and Mac OS X allow you to perform a clean install when upgrading your operating system. The installer will give you the choice between a standard upgrade (typically the default option) and a clean installation near the beginning of the installation process. Windows 7 also lets you format and partition your installation disk if you select a clean install. In Mac OS X, you can use the Disk Utility program to format or partition your drive before you perform the clean install.
NOTE: Since a clean install wipes out all data on the primary hard disk, it is crucial to back up your data before performing the installation. While it is wise to make sure you have a recent backup of your data before any OS upgrade, it is extremely important when performing a clean install. Backing up your data to an external hard drive or another computer are both good options. It is also smart to check your backup and make sure it contains all the files you need so you do not accidentally lose any files.