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Why did it take so long for Microsoft to include effective, free antivirus software as part of Windows? Blame the 2001 United States versus Microsoft antitrust settlement, which severely restricted the company's ability to bundle software with Windows if that software would compete with third-party products.
Through the decade, Microsoft slowly introduced various antimalware solutions. Windows Live OneCare was a paid product, and Windows Defender (included free with Windows Vista) blocked only adware and spyware.
Microsoft Security Essentials was the first free full-strength security product from Microsoft, based on the same engine as the enterprise-grade Forefront product. Its successor will be included by default in all editions of Windows 8, using the well-established Windows Defender brand name.