http://www.microsoft.com/technet/windowsvista/evaluate/feat/secfeat.mspx

Security threats evolve constantly. To stay protected from threats on the Internet and wireless networks, the Microsoft Windows client operating system must also evolve. Windows Vista is the most secure and trustworthy Windows operating system yet, and it will help organizations achieve their business and computing goals with confidence. This paper describes the most significant security improvements, the benefits they provide, and why the new features matter to IT professionals.