type of computer program that is secretly installed on a person's computer in order to divulge the owner's private information, including lists of World Wide Web sites visited and passwords and credit-card numbers input, via the Internet.

Spyware typically finds its way onto users' computers when they install some other software, such as electronic games (electronic game) or system utilities, from third-party sources that have altered the original programs. For example, a large proportion of software downloaded from P2P (“person-to-person”) file-sharing (Internet) networks contains computer viruses, worms, spyware, adware (unsolicited advertisements), or other “malware.” Spyware may also be secretly installed when a user opens an infected e-mail attachment. Because digital audio or video files are frequently shared among friends, a contaminated file can quickly proliferate if left unchecked.

Some spyware is designed to steal U.S. Social Security numbers, passwords, and other private information directly from an infected computer's hard drive, while other spyware may alter the results of Internet searches in order to redirect users to a Web site that may infect their computers with even more spyware. Most commercial antivirus software programs include features to help detect and eliminate spyware and other malware. In addition, modern operating systems (operating system) include features to make it harder for criminals to install malware without the owners' knowledge. Still, all such preventative measures are ineffective if users do not regularly update their system and antivirus software, and no combination of security measures will work if individuals indulge in imprudent behaviour.