I have been using it more then 6 months, seems to be pretty good.. 

Microsoft Windows AntiSpyware (Beta) is a security technology 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. Participants in the worldwide SpyNet™ community play a key role in determining which suspicious programs are classified as spyware. Microsoft researchers quickly develop methods to counteract these threats, and updates are automatically downloaded to your PC so you stay up to date.

Get you copy http://www.microsoft.com/athome/security/spyware/software/about.mspx