Windows Defender is an inbuilt tool into Windows Vista and available as a free download from Microsoft for XP users.
Windows Defender is easy to use and lets you do manual, scheduled, and real-time scanning with automatic updates of spyware signatures, without a large performance penalty.
If you have already installed or using a third-party anti-spyware application, there's really no need to keep Windows Defender's real-time scan enabled. In this case you can get a performance increase by disabling it.
Using Windows Defender
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