Windows Defender ( Beta 2 )

Windows Defender (Beta 2) is a free program that helps protect your home computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software.
It features Real-Time Protection, a monitoring system that recommends actions against spyware when it’s detected, and a new streamlined interface that minimizes interruptions and helps you stay productive.
Get more information :

http://www.microsoft.com/athome/security/spyware/software/default.mspx