boot from vhd is a great new feature of windows 7 enterprise.  it allows you to start an operating system configured in vhd file.  i have been using this to run 64 bit builds of windows 7 on a system that is natively running the 32 bit version.  how do you create the vhd?  install windows 7 in hyper-v or virtual pc, sysprep it and then save it or use the wim2vhd tool from msdn here.  here are the steps for setting up boot from vhd.

copy a sysprepped vhd into a directory (for this example, we’ll use the c:\vhd directory and a file called win7-64.vhd)

from an elevated command prompt type:

bcdedit /copy {current} /d "windows 7 64 bit"

the output from this command provides you a guid that you will use in the next three commands.  replace the %guid% with the actual guid you received from the first command above.

bcdedit /set %guid% device vhd=[locate]\vhd\win7-64.vhd

bcdedit /set %guid% osdevice vhd=[locate]\vhd\win7-64.vhd

bcdedit /set %guid% detecthal on