Effects and behaviors

A piece of spyware rarely "lives" alone: an affected computer can rapidly become infected with large numbers of spyware components. Users frequently notice unwanted behavior and degradation of system performance. A spyware infestation can create significant unwanted CPU activity, disk usage, and network traffic — slowing down legitimate uses of these resources. Stability issues — application or system crashes — are also common. Spyware which interferes with the networking software commonly causes difficulty connecting to the Internet.

The cumulative effect, and the interactions between spyware components, typically cause the stereotypical symptoms reported by users: a computer which slows to a crawl, overwhelmed by the many parasitic processes running on it. Moreover, some types of spyware disable software firewalls and anti-virus software, and/or reduce browser security settings, thus opening the system to further opportunistic infections, much like an immune deficiency disease. Documented cases have also occurred where a spyware program disabled other spyware programs installed by its competitors.

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Many spyware programs reveal themselves visibly by displaying advertisements. Some programs simply display pop-up ads on a regular basis — for instance, one every several minutes, or one when the user opens a new browser window. Others display ads in response to specific sites that the user visits. Spyware operators present this feature as desirable to advertisers, who may buy ad placement in pop-ups displayed when the user visits a particular site. It is also one of the purposes for which spyware programs gather information on user behavior.

Pop-up advertisements lead to some of users' most common complaints about spyware. A computer can become overwhelmed downloading or displaying ads. An infected computer rarely has only one spyware component installed — they more often number in the dozens — and so while a single program might display ads only infrequently, the cumulative effect becomes overwhelming.

Many users complain about irritating or offensive advertisements as well. As with many banner ads, many spyware advertisements use animation or flickering banners designed to catch the eye — thus they become highly visually distracting. Pop-up ads for pornography often display indiscriminately, including when children use the computer — possibly in violation of anti-pornography laws.

A further issue in the case of some spyware programs has to do with the replacement of banner ads on viewed web sites. Spyware which acts as a web proxy or a Browser Helper Object can replace references to a site's own advertisements (which fund the site) with advertisements which instead fund the spyware operator. This cuts into the margins of advertising-funded Web sites.