a-squared Free

You can also choose to first put software it finds into a quarantine, to make sure that it in fact is malware, before deleting it. This program digs deep -- not only will it find and kill spyware, but it will even detect "traces" such as Registry entries, and kill them as well.

a-squared doesn't offer real-time spyware protection, and won't protect your PC against getting infected in the first place; for that you'll need another piece of anti-spyware. But it's free, and so is ideal to use as a second or third spyware killer on your PC.

--Preston Gralla