Programs designed to launch advertisements, often pop-up banners, on host machines and/or to re-direct search engine results to promotional web sites. Adware programs are often built into freeware or shareware programs, where the adware forms an indirect ‘price’ for using the free program. Sometimes a Trojan silently downloads an adware program from a web site and installs it onto a user’s machine. Or hacker tools, often referred to as Browser Hijackers (because they subvert the web browser to install a program without the user’s knowledge), download the adware program using a web browser vulnerability.

Browser Hijackers may change browser settings, re-direct incorrect or incomplete URLs, or change the default homepage. They may also re-direct searches to ‘pay-to-view’ (often pornographic) web sites.

Typically, many adware programs do not show themselves in the system in any way: no listing under Start | Programs, no icons in the system tray, nothing in the task list. In addition, adware programs seldom come with a de-installation procedure and attempts to remove them manually may cause the original carrier program to malfunction.