You cannot uninstall Windows Vista, but you can reinstall your previous operating system, which then replaces Windows Vista. To reinstall your old operating system, you must have the original installation disc. Back up your programs and files, insert the installation disc into your computer, and then reinstall the old operating system. To regain the hard disk space used by Windows Vista, reformat the disk during the reinstallation process. After reinstalling your previous operating system, reinstall your programs and files.