Hello Polymer and welcome to the Forum.
I'm guessing you're talking about the .vmcx files that appear in the Virtual Machines folder. In actuality, .vmcx files are just shortcuts to .vmc files. It's my understanding that by default, Windows 7 places the .vmc files under the hidden AppData folder, but they can be anywhere. If you open a .vmc file you have saved elsewhere, Windows will automatically create the .vmcx shortcut under the Virtual Machines folder for you. So I don't think editing a .vmcx file will accomplish anything other than breaking the shortcut.
Some people have accidentally "lost" the XP Mode from the VM folder by deleting XPMode.vmcx. If this has happened you can find and double-click the XPMode.vmc file to recreate the XP Mode.vmcx file. The.vmc file is %systemdrive%\Users\<username>\ AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines, unless a different folder was selected during setup. In that case, search for “Windows XP Mode.vmc”. Slick IT: Windows 7 Virtual PC Management Troubleshooting Windows Virtual PC