I just found out how to set the default virtual machine that is launched via “Start > All Programs > Windows Virtual PC > Windows XP Mode”. You have to enable the ability to see hidden files, then navigate to "<user account directory>\AppData\Local\Microsoft\Windows Virtal PC" and edit the options.xml fie.
In my case, I had the following entry:
<default_vm type="string">Windows XP Mode 1</default_vm>
You just have the change the value to the virtual machine name you want to launch by default (i.e. "Windows XP Mode") and save file.
Edited by TitanFan, 20 August 2010 - 11:16 PM.