There are circumstances that require a fresh install, such as when a system becomes infected with a rootkit
. Rootkits can infiltrate the operating system
in such a way as to make removal problematic if not impossible. The only way to be sure of eradicating a rootkit is to reformat the drive, destroying all data. Once the drive has been reformatted you can reinstall Microsoft® Windows™ using the compact disc
that came with the machine, or a purchased retail version.