You can install the Windows Vista / 2008 / 7 bootloader onto a Windows XP machine and dual-boot Windows XP and Windows 2008 / 7 from the VHD (I don't believe Vista has VHD boot support). I'm doing this on my main laptop computer right now. The easiest way to do it is simply to install Windows 2008 / 7 into a VHD, and you get the bootloader "for free" during the install.
- Boot the Windows 7 DVD
- Choose Repair and Command Prompt
- From the command prompt, execute the following:
CREATE VDISK FILE="filename for virtual disk here" TYPE=vdisk MAXIMUM=size-in-megabytes
- Close up the command prompt. start the Windows 7 install.
- Choose the virtual disk (you'll be able to tell by size which one it is). You receive a warning that you cannot install onto that disk. Continue anyway and the install completes fine.
After that completes you will have Windows 7 install that can boot off of VHD and the Vista / 2008 / 7 bootloader.