Symptom

When you click Start Menu, the Turn Off Computer option may be missing. This is due to a group policy / registry restriction. To restore the missing option, try this - for stand-alone systems:

Resolution

NOTE: Backup the Registry before modifying.

Open Registry Editor by typing REGEDIT.EXE in the Start, Run dialog.

Navigate to the following locations:
HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies \ Explorer

-and-

HKEY_LOCAL_MACHINE \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies \ Explorer

In the right-pane, double-click the value NoClose and set it to 0.

Restart Windows for the changes to take effect