Less than a generation ago, computer viruses were considered an urban myth. They were found more often in movies than on actual computer systems. Now, however, malicious software constitutes a material threat to businesses, government, and home computer users.

Currently, there are three categories of malicious software threats: viruses, worms, and Trojan horses. All of these threats are built from the same basic instructions and computer logic that make up application programs on one's computer such as word processors, games, or spreadsheets. Like traditional application programs, malicious software is written by people and must be intentionally designed and programmed to self-replicate or cause damage.

While almost all Trojan horses attempt to cause harm to the computer system, more than 70 percent of all computer viruses and worms are designed only to self-replicate. Those viruses, worms, and Trojan horses that do inflict intentional damage to computer...