Why does Windows 7's Virtual PC running Windows XP Professional have no "Shut down" button?

"the shutdown command in XP isn't really removed, just hidden.

use the Group Policy editor (Gpedit.msc) under the following location to configure Start menu features:

User Configuration \ Administrative Templates \ Start Menu and Taskbar

Policy:Disable and remove the Turn Off Computer button
Description:Removes the ""Turn Off Computer"" button from the Start Menu and
prevents shutting down Windows using the standard shutdown user interface.

or check this registry key:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer

Registry Value (DWORD):""NoClose"" (1 = no Shutdown)

if you're not comfy with either option, you may create a shortcut to shutdown.exe and pin it to the start menu.

for possible switches, run this from a command prompt:

shutdown.exe /?"