yeah, i call them kill disks... do you mean floppy disks, right?
what those do is they overwrite everything on your hard drive X number of times... which make the data unrecoverable. Theoretically you could make a virus that will still be "alive" by simply reformatting.
Think of it this way. You take a pencil and write the word "DISK" with normal hand writting (not too light). "Erasing" it makes it still visible, which is like reformatting. Taking the pencil, and overwriting it with scribbles and such, makes it almost impossible to find out what you wrote.
but theoretically, you could also make your ocmputer into a robot...... heh, im gonna make a virus that turns your computer into a robot that when u put in a CD, it cuts it up and uses it as a saw... then the hard drives are wheels and it attacks you with it
...theory is not everything