Microsoft Windows Defender (also known as Windows AntiSpyware) is a security technology free software that helps protect Windows users from spyware and other potentially unwanted software.  
Known spyware on your PC can be detected and removed. This helps reduce negative effects caused by spyware, including slow PC performance, annoying pop-up ads, unwanted changes to Internet settings, and unauthorized use of your private information.  
Continuous protection improves Internet browsing safety by guarding more than 50 ways spyware can enter your PC.  
Requires Windows Validation. And Service Pack 2 (for WinXP & 2003).