Just an FYI, if you are trying to put an existing VM in Virtual PC 2007 and the VM was created on your windows 7 machine using Virtual PC; you may get an error. I received this error anytime I tried attaching an existing virtual hard disk to a new VM in virtual pc 2007.
“The parent virtual hard disk has been deleted or has been moved from its previous location. Please, select the new location of the parent virtual hard disk”
I found the solution to it. What it looks like is in windows 7 relies on a parent disk. It relies on Windows XP mode\Windows XP Mode base.vhd
So, if you just copy that file from your windows7 machine and copy it to the machine using Virtual pc 2007, it will work.
Also in virtual pc 2007, if you are trying to add a VM that is a .vhd file; you have to create a new VM and then add an existing virtual hard disk. It will let you use the .vhd file there.