Windows defender - anti spywere

Microsoft made their own anti spyware tool called Windows defender.
They say good thing about it, so read their review:

Windows Defender (Beta 2) is a free program that helps protect your 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.