Performing a clean installation of Windows 7, whether on a new computer or an old one, is a major job which will probably take you half of the day. On a new computer which does not come with an operating system preinstalled, you'll need to install Windows 7 (or another operating system) in order to use the computer. Fortunately, most new computers already come with Windows 7 preinstalled, in which case setting it up does not take very long. On an older computer, making a clean install of Windows 7 is a great way to get it up and running like new again. Over time, computer systems inevitably develop problems and get bogged down with unused data and software. There comes a point when the most effective way to optimize your computer and to fix problems is to start from scratch and install Windows 7 on a formatted hard drive. This also ensures that any malicious software is wiped out.