Advertisements or Spyware

Many spyware programs display 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.

Many users complain about irritating or offensive advertisements as well. As with many banner ads, many spyware advertisements use animation or flickering banners which can be visually distracting and annoying to users. Pop-up ads for pornography often display indiscriminately. Links to these sites may be added to the browser window, history or search function. When children are the users, this could possibly violate anti-pornography laws in some jurisdictions.

A further issue in the case of some spyware programs has to do with the replacement of banner ads on viewed web sites. Spyware that 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 that instead fund the spyware operator. This cuts into the margins of advertising-funded Web sites.

http://en.wikipedia.org/wiki/Spyware