According to a TechNet article about deploying Windows Vista in enterprise environments, there will be a version of Windows Vista released that is bundled with Microsoft's Virtual PC application that can be used for application compatibility. There is no word on if the Virtual PC engine will be integrated more deeply into the operating system to allow for faster performance of virtual machines.
From the Article, "Some versions of Windows Vista even include a version of Microsoft Virtual PC to enable users to run a different operating system in a window on their computer."
View: Full article @ TweakVista
