windows sidebar is managed by your administrator

• Start -> type "regedit" in the Search and open the Registry Editor.
• Browse to: - "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Windows\Sidebar"
• Right-click on "turnoffsidebar" and change 1 to 0.

Another way to enable the sitebar

•  In the command prompt, type regsvr32 msxml3.dll and press Enter
  
• In the command prompt, type regsvr32 scrrun.dll and press Enter.
  
• In the command prompt, type regsvr32 jscript.dll and press Enter.