Keith Combs wrote an excellent article about the How to boot from vhd in windows 7 and run Windows 2008 R2 with Hyper-v.
If you are running Windows 7 today, there’s no reason to tear down your machine, re-partition your drive, and reinstall your world. Instead, you can simply install Windows Server 2008 R2 into a .VHD file and make it bootable. After that is accomplished you can choose the OS you want when you power up your machine. The following steps are the simplest way (currently) to do this. It really doesn’t get any simpler.
Read more here.