Source : Free On-Line Dictionary of Computing

boot virus
     
        An {MS-DOS} {virus} that infects the {boot record} program on
        {hard disk}s and {floppy disk}s or the {master boot record} on
        hard disks.  The virus gets loaded into memory before {MS-DOS}
        and takes control of the computer, infecting any floppy disks
        subsequently accessed.  An infected {boot disk} may stop the
        computer starting up at all.
     
        (1995-02-16)