In the past 20 years, the Microsoft Windows operating system has accumulated old code libraries that brought it to the size it has today, 2.5 GB and about 50 million lines of code (Windows Vista). These old code libraries consume resources and are often the targets of security exploits. The best way to avoid such problems, is to start from scratch, which is close to what Microsoft plans to do with Windows Vienna. Windows Vienna will represent the start of a different generation of operating systems, bringing in new concepts and support for new types of hardware, along with a better security and a modular approach, which will allow future versions of Windows to be built more easily on Windows Vienna's engine.
It is also likely that the future success of Microsoft's products will be strongly decided by the success of the new generation operating system.