Adware has been criticized for occasionally including code that tracks a user's personal information and passes it on to third parties, without the user's authorization or knowledge. This practice has been dubbed spyware and has prompted an outcry from computer security and privacy advocates, including the Electronic Privacy Information Center.

Noted privacy software expert Steve Gibson of Gibson Research explains: "Spyware is any software (that) employs a user's Internet connection in the background (the so-called 'backchannel') without their knowledge or explicit permission. Silent background use of an Internet 'backchannel' connection must be preceded by a complete and truthful disclosure of proposed backchannel usage, followed by the receipt of explicit, informed consent for such use. Any software communicating across the Internet absent of these elements is guilty of information theft and is properly and rightfully termed: Spyware."

Spyware has surpassed viruses as the number one threat facing your computer today. In fact, most estimates report that 90 percent of computers already have been infiltrated by spyware, which is the name given to any program that installs itself on your hard drive, collects personal information about you and your computer habits, then sends it to a third party without your knowledge.

Spyware gets into your system in several ways. It can come bundled with another program and automatically install along with the other software. Spyware also can get a foothold on your system through the advertising used in some free software, such as peer-to-peer programs like Morpheus and Kazaa. This type of spyware is known as adware.

Another method of entry occurs when you visit certain Web sites that install spyware cookies on your hard drive. While many cookies are completely harmless, others function like adware and gather details about the sites you visit. Also, a malicious person can intentionally install spyware on your computer. These programs, called keyloggers, record every site you visit, every e-mail you send and every chat conversation you carry on.

For the average user, any of these spyware variants would be hard to detect and even more difficult to remove.