anti-malware

Best free

Windows Defender
http://www.microsoft.com/windows/products/winfamily/defender/default.mspx

Spy-bot
http://www.safer-networking.org/en/download/index.html

Adaware free
http://www.lavasoft.com/products/ad_aware_free.php

Malwarebytes' Anti-Malware 1.36
http://www.pcmag.com/article2/0,2817,2345349,00.asp

a-squared free
http://www.emsisoft.com/en/software/free/

Best commercial

Spyware Doctor
http://www.pctools.com/

Spyware Sweeper
http://www.webroot.com/En_US/consumer-products-spysweeper.html