Windows 8 is said to be a complete ground-up rebuild of the Microsoft Windows OS. Not only is the UI undergoing a radical change, but the entire structure of the Windows OS has been completely redesigned and as such is expected to break current Windows drivers and applications.
While the software emulation of the Xbox 360 hardware would definitely run slower on a PC, Microsoft may be counting on the increased power of modern graphics cards and GPUs to make up the performance difference. The Xbox 360 itself is running on a stripped down version of the NT kernel and the onboard GPU is DirectX 9 capable. As long as Microsoft writes a software emulator for the power PC derived CPU, the graphics side should be relatively easy to solve.
Further, we also know that the Xbox 360 controller has been capable of being used on Windows PCs for quite some time, and with the recent announcements that Kinect will also be supported by Windows, we've got all the gaming specific hardware pieces needed to run Xbox 360 software on a PC.
As to why Microsoft might do something like this, it is an easy way to leverage the existing Xbox 360 gaming library to increase software sales. By turning any compliant PC capable of running Windows 8 into a virtual console, Microsoft can tap a far larger market than that which exists for the Xbox 360.
In fact, this is quite ingenious since Microsoft wouldn't even need to subsidize the hardware cost of the console. This means a larger portion of the licensing fee that is charged to the software makers goes directly to Microsoft's bottom line. Furthermore, Microsoft could even enable streaming of games over a local area network from a desktop PC to Microsoft Mobile phones and tablets running the Windows 8 OS - similar in concept to the OnLive service.
Rumors and conjectures aside, everything should be taken with a grain of salt. The possibility is certainly there, but it remains to be seen if Microsoft will actually make the effort to realize this potential.
Source: Redmond Pie via Insideris