The concept of “adware” includes programs that run ads (often in the form of pop-up windows) or redirect search requests to advertising sites.
Adware is often built into freeware or shareware programs, and installed on your computer at the same time with the main application without knowledge or consent. Sometimes adware can secretly download and install on your computer Trojan horse.
Obsolete, not updated on time versions of web browsers may be vulnerable to hacker tools, download adware to your computer. There is also a “program-browser hijackers” that can change the settings of a browser redirect incorrectly typed or incomplete URL-address-specific Web sites or to change the set to the home page. They can also redirect searches to pay (often pornographic) websites.
Advertising programs usually do not manifest themselves in the system: they are not visible in the list of programs on the tab Start -> Programs, there are no appropriate icon in the notification area of the system and in the task list. For rarely has a procedure uninstall (remove) the attempt to remove them by hand can lead to malfunction host program (which included advertising program was installed).
Related post:
Symptoms of infection with the malware
That to do to me if my computer is infected
MALWARELIST.net - Your Information Security Source 