To enable GPO startup / shutdown / log-on / log-off logging:

• Open registry editor (regedit.exe) and navigate to:
  HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Diagnostics
  Create key “Diagnostics” if it does not exist.
• Create new DWORD Value called GPSvcDebugLevel and set value (HEX) to 0x30002
• Restart the PC

Windows 7