Each Windows user wants to have the ability to set up and use a PC in a way that is unique to them. Doing so, however, can be challenging in today’s multiple user and multiple PC environment. We know that shared PC usage is common and we’ve heard from many of you that switching between multiple accounts can be cumbersome. The difficulties associated with managing multiple accounts often lead to the sharing of a single account on a PC, and a less personal (and potentially less private) experience for each user. We also know that users are utilizing multiple devices more often now, and setting up a new PC can be inconvenient and time consuming. In Windows 8, we have set out to ensure that each PC user has a truly personal experience that seamlessly bridges their online and offline tasks, is simpler to set up and use, and persists across their set of Windows 8 PCs. To do this, we’ve introduced the ability to log in to Windows (optionally) with a Windows Live ID that works across devices, apps, and services, allowing you a uniquely personal experience with Windows.
http://blogs.msdn.com/b/b8/archive/2011/09/26/signing-in-to-windows-8-with-a-windows-live-id.aspx
