Anti-virus programs are designed to detect, delay, and respond to programs or pieces of code that are specifically designed to harm computers. These programs are known as "malware." Malware can include computer viruses, worms, and Trojan Horse programs (programs that appear to be benign but which have hidden harmful effects).

Pest eradication tools are designed to detect, delay, and respond to "spyware" (strategies that websites use to track user behavior, such as by sending "cookies" to the user's computer), and hacker tools that track keystrokes (keystroke loggers) or passwords (password crackers).

Viruses and pests can enter a computer system through the internet or through infected floppy discs or CDs. They can also be placed onto a system by insiders. Some of these programs, such as viruses and worms, can then move within a computer's drives and files, or between computers if the computers are networked to each other. This malware can deliberately damage files, utilize memory and network capacity, crash application programs, and initiate transmissions of sensitive information from a PC. While the specific mechanisms of these programs differ, they can infect files, programs, individual pieces of computer code, operating systems, and even the basic operating program of the computer firmware/hardware.

The most important features of an anti-virus program are its abilities to identify potential malware and to alert a user before infection occurs, as well as its ability to respond to a virus already resident on a system. Most of these programs provide a log so that the user can see what viruses have been detected, and where they were detected. After detecting a virus, the anti-virus software may delete the virus automatically, or it may prompt the user to delete the virus. Some programs will also fix files or programs damaged by the virus.

Various sources of information are available to inform the general public and computer system operators about new viruses being detected. Since anti-virus programs use signatures (or snippets of code or data) to detect the presence of a virus, periodic updates are required to identify new threats. Many anti-virus software providers offer free upgrades that are able to detect and respond to the latest viruses.

The cost of anti-virus software packages varies depending on the complexity and level of protection provided (i.e., some anti-virus software may be able to respond to new viruses because they respond to certain patterns in the computer code). Shareware (software that can be downloaded for free from the internet) anti-virus programs can be obtained via the internet free-of-charge. These free programs provide a basic level of protection. Individual PC anti-virus programs, which provide additional virus protection, range from $25 to $60. Network versions for a typical system of 10-25 computers range from $250 to $1000 or more. Installation and maintenance costs will vary depending on the ease-of-use, setup and configuration. Installation time ranges from a few hours for an individual PC to 8-40 hours or more of man-hours for a small to medium size network of workstations. Installation time varies depending on the level of computer software and hardware skills of the installer. Ongoing maintenance of anti-virus systems typically requires computer support time of several hours or more per month.