Virtual Hard Disk allows you to create, mount, and unmount OS images and Microsoft did a wonderful job by providing a native support in Win7. This allows users to use virtualization on Win7 without any additional software, a turf that VMware cannot play! The guest OS residing in VHD can be booted directly without the host OS. Microsoft have effectively created a "loopback HBA", and the bootloader can use to address VHD's as any regular disk. I used this on my PC and admit that this feature works rock solid on Win7, though still in beta phase.
This could be a game changing technology, since one doesn't need additional software (like VirtulBox or VMware Workstation) to use Virtualization for booting multiple OS. This gives new abilities to test even a native device driver in the guest OS as it works just as the host OS. Realize that Win2008 Server also supports this feature, but the kernel implementations of Win2008 and Win7 are totally different!
In order to use this feature, just know to use the specifics of Win7 VHD commands diskpart, bcedit and you are good to go.
