According to the release notes: "Windows OneCare will not be compatible with the Windows Vista operating system (formerly codenamed ‘Longhorn’) when that operating system is released."
With Microsoft’s big emphasis on security in Windows vista, OneCare incompatibility is tough to rationalize. Why should Microsoft’s major new consumer software product be incompatible with its major new operating system? One explanation makes sense: That Microsoft will bundle similar capabilities found in Windows OneCare, including antivirus, into Windows Vista."
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I find it strange myself. I say the more security, the better, especially for products such as AntiVirus which Windows cannot do with out.