I was wondering if it is possible to boot a Windows XP VHD from Windows 7. I know it is possible to boot a Windows 7 VHD or a Windows Server 2008 VHD but i'd like to do so with an older O/S. Is there any known workaround for this? Thanks JP
Unfortunately, no (assuming you are referring to the boot-the-whole-computer-from-a-vhd feature of Windows 7). You can of course boot a Windows XP VHD from Virtual PC, Virtual Server, Hyper-V, VirtualBox, VMWare...
The changes needed to add that functionality were made at the kernel level, and are not available prior to Windows 7 or W2K8 R2.
More information can be found here: Frequently Asked Questions: Virtual Hard Disks in Windows 7