Antispy Software, article by Eran Shaham, security expert

Spyware is computer software that collects personal information about users without their informed consent. The term, coined in 1995 but not widely used for another five years, is often used interchangeably with adware and malware (software designed to infiltrate and damage a computer respectively).

Personal information is secretly recorded with a variety of techniques, including logging keystrokes, recording Internet web browsing history, and scanning documents on the computer's hard disk. Purposes range from overtly criminal (theft of passwords and financial details) to the merely annoying (recording Internet search history for targeted advertising, while consuming computer resources). Spyware may collect different types of information. Some variants attempt to track the websites a user visits and then send this information to an advertising agency. More malicious variants attempt to intercept passwords or credit card numbers as a user enters them into a web form or other applications.

The spread of spyware has led to the development of an entire anti-spyware industry. Its products remove or disable existing spyware on the computers they are installed on and prevent its installation. However, a number of companies have incorporated forms of spyware into their products. These programs are not considered malware, but are still spyware as they watch and observe for advertising purposes. It is debatable whether such 'legitimate' uses of adware/spyware are malware since the user often has no knowledge of these 'legitimate' programs being installed on his/her computer and is generally unaware that these programs are infringing on his/her privacy. In any case, these programs still use the resources of the host computer without permission.

How to choose the best antispy program?

1. Use it for free as a windows genuine product customer, as a windows operating system add-on or a built in component. 
   Recently Microsoft acquired GIANT’s antispyware software, rebranding it as Windows Antispyware beta and releasing it as a free download for Windows XP and Windows 2003 users. In early spring, 2006, Microsoft renamed the beta software to Windows Defender, and it was released as a free download in October 2006. Microsoft has also announced that the product will ship (for free) with Windows Vista.

Or, Use it as a part of your antivirus program. 
Major anti-virus firms such as Symantec, McAfee and Sophos have come later to the table, adding anti-spyware features to their existing anti-virus products. However, recent versions of these major firms' home and business anti-virus products do include anti-spyware functions, albeit treated differently from viruses. Symantec Anti-Virus, for instance, categorizes spyware programs as "extended threats" and now offers real-time protection from them (as it does for viruses).

1. Keep your antispyware software up-to-date. 
   Like most anti-virus software, many anti-spyware/adware tools require a frequently-updated database of threats. As new spyware programs are released, anti-spyware developers discover and evaluate them, making "signatures" or "definitions" which allow the software to detect and remove the spyware. As a result, anti-spyware software is of limited usefulness without a regular source of updates. Some vendors provide a subscription-based update service, while others provide updates for free. Updates may be installed automatically on a schedule or before doing a scan, or may be done manually.
2. Remember that if you are unsure about the program, don't install it. A free program is sometimes expensive in terms of the spy battle!