How Spyware Affects and Behaves

A spyware program is rarely alone on a computer: an affected machine can rapidly be infected by many other 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, all of which slow the computer down. Stability issues, such as application or system-wide crashes, are also common. Spyware which interferes with networking software commonly causes difficulty connecting to the Internet.

In some infections, the spyware is not even evident. Users assume in those situations that the issues relate to hardware, to Windows installation problems, or a virus. Some owners of badly infected systems resort to contacting technical support experts, or even buying a new computer because the existing system "has become too slow". Badly infected systems may require a clean reinstallation of all their software in order to return to full functionality.

Only rarely does a single piece of software render a computer unusable. Rather, a computer is likely to have multiple infections. As the 2004 AOL study noted, if a computer has any spyware at all, it typically has dozens of different pieces installed. The cumulative effect, and the interactions between spyware components, cause the symptoms commonly 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. Some spyware has disabled or even removed competing spyware programs, on the grounds that more spyware-related annoyances make it even more likely that users will take action to remove the programs. One spyware maker, Avenue Media, even sued a competitor, Direct Revenue, over this; the two later settled with an agreement not to disable each others' products.

Some other types of spyware (Targetsoft, for example) modify system files so they will be harder to remove. Targetsoft modifies the "Winsock" Windows Sockets files. The deletion of the spyware-infected file "inetadpt.dll" will interrupt normal networking usage. Unlike users of many other operating systems, a typical Windows user has administrative privileges, mostly for convenience. Because of this, any program the user runs (intentionally or not) has unrestricted access to the system too. Spyware, along with other threats, has led some Windows users to move to other platforms such as Linux or Apple Macintosh, which are less attractive targets for malware. This is because these programs are not granted unrestricted access to the operating system by default, though some allege it's mainly due to the far smaller number of machines installed with these operating systems making spyware development potentially less profitable for these platforms.[citation needed]. As with other operating systems, Windows users too are able to follow the principle of least privilege and use non-administrator least user access accounts, or to reduce the privileges of specific vulnerable Internet-facing proceses such as Internet Explorer (through the use of tools such as DropMyRights). However as this is not a default configuration, few users do this.

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. Pop-ups are one of users' most common complaints about spyware.

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.