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Spyware refers to programs that install themselves without permission and monitor anything from browsing habits to passwords and even adware programs usually perform some kind of tracking for market research.
Some adware can be installed unintentionally along with other applications. Purveyors of adware exploit the assumption that most people will not read the fine print. File-sharing programs, some free games and utilities are notorious for installing aware.
Some adware and spyware can be installed without permission when you visit web sites. This code can hijack the web browsers and change the default home page while installing adware. The best way to prevent this is keeping your anti-virus software up-to-date and running weekly or daily scans. For more information and links to help prevent and find adware software go to: http://wiki.colgate.edu/itwiki/AcademicTechnology:STRG/PublicPage#Software_Tools