SpyBot Search and Destroy and Microsoft Windows Defender are definitely the best out there, even topping all the paid ones. But the key is to not get the spyware on your computer in the first place because its getting more and more difficult to remove without resorting to a full format and reinstall.
The best way to prevent the spyware infestation is to use the Firefox or Opera web browsers instead of Internet Explorer and only download software from very trusted sources. You could also use SpywareGuard, SpywareBlaster, and SpyBot's Immunize feature to help prevent things from getting in.
Just make sure you use the update feature on these programs several times of week or they won't be able to stop the the new stuff.
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