Microsoft Windows AntiSpyware
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Full user review
It offers real-time protection based upon "community report," various rules, and known characteristics. It's scanner can be set to run automatically, and found numerous entries that both AdAware and SpyBot missed. It even alerts you when programs are trying to change your settings, and allows you to restore them. Also, its definitions are updated every week, unlike many of the alternatives.
It's only a BETA and it's already convincing me to uninstall its competitors. Hopefully when the full version comes out this summer, it will remain free, or at least be low-priced.