These destructive programs may start out on one computer, but they very quickly get copied and sent to other computers around the world. But keep in mind that most personal computer viruses are spread through floppy discs or by running infected programs. In fact, the most danger to a public access computer is from the floppy disk used by one patron and forgotten in the floppy drive. That disk might contain a virus that then infects the public access computer if it is rebooted while the forgotten disk is in the drive. See the page CMOS Security Settings Passwords, Virus protection, and Boot Sequence for information on reversing the boot sequence to avoid this problem.
Macro viruses are another class of virus that can also be very destructive.
Any program such as a word processor or spread sheet that provides a macro programming language could possibly be a target of a macro virus. With the prevalence of Microsoft Word in the world today, macro viruses may be the most common type of virus. Since macros may be required to even view a document, it is impractical to completely disable the macro feature of your word processor. Therefore, the only protection you really have is to use a good quality virus scanning program. All major virus scanning software vendors provide free updates on their Web sites.