Many users view Windows XP (and Windows 2000, and previous Windows versions) as unsafe. No matter how many patches and updates Microsoft releases, the foundation of the OS itself—the kernel—is designed and built in a way that prevents it from being truly secure. The only solution, it is argued, is to redesign and rebuild the kernel with a focus on security and stability.
Well, thats exactly what Microsoft is doing with Vista. The whole kernel has been reorganized and rewritten to help prevent software from affecting the system in unsavory ways. In Vista, it should be much more difficult for unauthorized programs (like Viruses and Trojans) to affect the core of the OS and secretly harm your system.
News source: ExtremeTech