Hi Bob, welcome to the Seven Forums.
First, when you told you tried to shut down XP Mode, did you mean really shut down or hibernate? I mean, hibernate is default XP Mode closing action if you have not changed it. If your closing action was hibernate, then there's a trick that might work for you.
Virtual PC vm's create a hibernation file when hibernated. The file name is <Name_of_the_vm>.vsv (vsv = virtual saved state) and it is by default saved in C:\Users\Your_Username\AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines
Check the above mentioned folder to see if you can find this hibernation file Windows XP Mode.vsv; it only exists if XP Mode is hibernated. If it exists, you can safely delete it. This causes XP Mode to boot instead of resume from hibernation when next time started.
Report back if this fails or if the .vsv file is not there.
Kari