Windows Defender has been released!
Yesteday, Microsoft released their Windows Defender program. Windows Defender is a free program that helps protect your computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software.
Remember that you cannot install the program on pre-XP OSs. Read the System Requirements and read the Release Notes before installing the Windows Defender program.
More information at: Get Started using Windows Defender.
You can download it at the Download Center.