A PC is infected with a boot sector virus (or partition sector virus) if it is (re-)booted (usually by accident) from an infected floppy disk in drive A. Boot Sector/MBR infectors are the most commonly found viruses, and cannot normally spread across a network. These (normally) spread by accident via floppy disks which may come from virtually any source: unsolicited demonstration disks, brand-new software (even from reputable sources), disks used on your PC by salesmen or engineers, new hardware, or repaired hardware. A file virus infects other files when the program to which it is attached is run, and so *can* spread across a network (often very quickly). They may be spread from the same sources as boot sector viruses, but also from sources such as Internet FTP sites and bulletin boards. (This applies also to Trojan Horses.) A multipartite virus infects boot sectors *and* files. Often, an infected file is used to infect the boot sector: thus, this is one case where a boot sector infector could spread across a network.