Spyware differs from viruses and worms in that it does not usually self-replicate. Like many recent viruses, however, spyware is designed to exploit infected computers for commercial gain. Typical tactics furthering this goal include delivery of unsolicited pop-up advertisements; theft of personal information (including financial information such as credit card numbers); monitoring of Web-browsing activity for marketing purposes; or routing of HTTP requests to advertising sites.
As of 2005, spyware has become one of the pre-eminent security threats for computers running Microsoft Windows operating systems. Some malware on the Linux and Mac OS X platforms has behavior similar to Windows spyware, but to date has not become anywhere near as widespread.
Spyware blockers and antivirus software are one of the best ways to protect your computer from security threats.
You can often find spyware blockers and antivirus software at a discount eBay. A software store may have gotten surplus copies of spyware blocker software or wish to reduce their inventory due to an updated version and will sell new copies of spyware blockers at a reduced cost. Before you bid on an auction, make sure that the spyware blocker product says the product is "new and in a retail box".