How can I install Windows 7 rtm vhd files in an existing windows xp machine?
If you are talking about VHD files, you do not install this. A VHD image is a complete installation of Windows that is ready to run in a virtual machine.
The easiest thing you can do in Windows XP is to download Microsoft Virtual PC 2007, and then simply copy all the VHD files to a directory and either create a new virtual machine pointing to the VHD or run the configuration file (It depends what you downloaded).
You should then be running Windows 7 virtually within your Windows XP installation.