Double check at the registry location below to see if you may have a
TurnOffSidebar DWORD entry. If you do, delete it.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Windows\Sidebar
You might also use the
Find and Find Next option in regedit to search for any
TurnOffSidebar DWORD entries other than at the location below, and delete any found.
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Windows\Sidebar