Windows Vista is not just a security update...it is the next version of Windows. Anyone who wants it will have to purchase the upgrade or full version, just like when Microsoft introduced Windows XP as the successor to Windows ME and Windows 2000. In addition, it will require "today's level processor" to run the latest version correctly, meaning those with computers 4+ years old will most likely have to buy a new computer if they want the latest OS.
However, this transition is a ways away, as it isn't to be officially released until late 2006 (it just began beta testing yesterday). In addition, Microsoft has announced that since Windows XP has had so much success, they will continue to support it for years to come, including porting a few Windows Vista (previously codename Longhorn) features into XP.
Hope this helps,
John

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