the best protection against viruses can be obtained by using a system that boots off a read-only medium, such as a compact disc (called cd), digital versatile disc (called dvd), or usb flash drive (called usb) that does not allow write access to hard disk (or other removable media). this limits the usability of the computer as it is not possible to save documents or install new programs. only -r media (not -rw) should be used, and the media should be finalized to prevent creating a new session that might possibly contain a virus.