Of course viruses can be spread through floppy disks, usb keys or other methods so being a STANDALONE computer not connected to any network doesn't mean the computer can not be INFECTED THOUGH the information cannot be LEAKED via the network to external persons. However, there is also physical security of the computer itself, and that where it gets interesting depending on who and what your trying to secure the pc from. If for instance the pc is sitting in a public area, and you are not worried just about external threats but also potential employee DATA theft then one should assume no information on the pc is secure even if the pc is standalone.

Of course viruses can be spread through floppy disks, usb keys or other methods so being a standalone computer not connected to any network doesn't mean the computer can not be infected though the information cannot be leaked via the network to external persons. However, there is also physical security of the computer itself, and that where it gets interesting depending on who and what your trying to secure the pc from. If for instance the pc is sitting in a public area, and you are not worried just about external threats but also potential employee data theft then one should assume no information on the pc is secure even if the pc is standalone.