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Microsoft's Windows 8 will activate its built-in antivirus software only if it senses that the PC is not protected by another security program, says McAfee.

From ComputerWorld:
The new operating system, which last week reached its final public milestone, includes security software, dubbed "Windows Defender," that combines characteristics of both that anti-spyware program of the same name and the free Security Essentials, the antivirus program that until now has been offered as a separate download.

Microsoft first announced Windows Defender last September, and included it with that year's Windows 8 Developer Preview, and this year's Consumer Preview and Release Preview. In all three sneak peeks, Defender was switched on, and served as the operating system's default protection against malware.
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