In general, Spyware? is any technology that aids in gathering information about a person or organization without their knowledge. On the Internet?, Spyware? is a program that is put in someone's computer to secretly gather information about the user and relay it to advertisers or other interested parties. Spyware? can get in a computer as a software Virus or as the result of installing a new program.
Data collecting programs that are installed with the user's knowledge are not, properly speaking, Spyware?, if the user fully understands what data is being collected and with whom it is being shared.

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Software that gathers information about a user as he or she navigates around the Web, it is intended to track surfing habits in order to build marketing profiles. Spyware? is often included in "free downloads" from the Web, where the license agreement (which so many of us accept without reading) may mention that information about your habits will be transmitted back to the company's Web site-but not information specifically about you. Spyware? is cause for public concern about privacy on the Internet.

This type of software generally only affects Windows systems; if you're running Linux or FreeBSD you'll never be bothered by large purple monkeys? or YellowLinks?.

Examples: RealPlayer, BonziBuddy?, Gator, Comet-cursor, HotBar?, WindowsMediaPlayer(latest version), WindowsXP, etc, etc.

BonziBuddy? and most other software of this type can be removed by Ad-aware. In some cases the Spyware? will try to remove Ad-aware first, and/or prevent lavasoft's download site from resolving so you might have to download it via an uninfected computer.
A number of other Spyware?-removers are available. For best results you are advised to use at least one or two other Spyware?-removers in addition to Ad-aware. (SpyBot - search&destroy, etc)

Spyware? will make your Windows installation unstable or destroy it all together. Fun.