Create a shortcut to your virtual machine and put it in the Startup Program group of the user you wish to have the Virtual Machine configuration applied to.
You may want to customise the shortcut to appear full screen and automatically boot (You can do this by right-clicking the shortcut and editing the Target field):
"C:\Program Files\Microsoft Virtual PC\Virtual PC.exe" -pc "Windows XP" -launch -fullscreen
A more complete reference to command-line options for Virtual PC is available online here:
In addition, you might want to go one further and disable Explorer from starting and using VirtualPC as the shell instead of starting the Windows Explorer. These instructions apply to Windows XP Embedded, but may well work with Windows 7, if you're considering experimentation: