The US based ISP Earthlink has published a report which shows that the average home computer is infected with 28 instances of spyware. Over a million scans were performed during the test, and over 29 million spyware instances were found.
Spyware or adware usually take the form of a program or cookie which tracks a users behaviour on a computer and sends statistics back to a central location for various reasons. It may be to help produce more targetted marketing, or for far more nefarious purposes such as keystroke logging to gain access to a users passwords. In most instances this software will slow down the end users machine.
Spyware arrives on a users PC in a variety of ways, usually without the knowledge of the end user. Cookies generally come as part of a web page, spyware software often comes packaged with freeware or shareware software. Often a long license agreement will hide the fact that you are agreeing to install third party software along with your desired application.
It's not all bad news however. Various companies already provide tools to protect and clean your computer, such as Lavasofts Ad-aware (a free version is available). Anti-virus companies are also expanding their tools to remove these products from the end users machine, such as Symantec's latest release of their popular Anti-virus tool Norton AntiVirus 2004.
As with viruses, spyware is constantly evolving and updating, so it's vital to ensure that any anti-spyware product you use is also kept up to date with new definitions files and so on. We recommend that to help protect your PC you equip yourself with some form of anti-spyware tool and run it reguarly, as you would an anti-virus tool.