Source: About.com

The best operating system ever offered for a personal PC, optimized for the individual. Windows Vista Ultimate Edition is a superset of both Vista Home Premium and Vista Pro Edition, so it includes all of the features of both of those product versions, plus adds Game Performance Tweaker with integrated gaming experiences, a Podcast creation utility (under consideration, may be cut from product), and online "Club" services (exclusive access to music, movies, services and preferred customer care) and other offerings (also under consideration, may be cut from product). Microsoft is still investigating how to position its most impressive Windows release yet, and is looking into offering Ultimate Edition owners such services as extended A1 subscriptions, free music downloads, free movie downloads, Online Spotlight and entertainment software, preferred product support, and custom themes. There is nothing like Vista Ultimate Edition today. This version is aimed at high-end PC users and technology influencers, gamers, digital media enthusiasts, and students.
Source: Microsoft
Modularization. Windows Vista is built with inter-dependent modules, which makes it easier to customize Windows Vista (to a certain degree) to your needs. Modularization also simplifies adding device drivers, testing and installing updates, and adding languages.
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Will Windows XP compatibelt with Windows Vista file system type?