Type regedit in RUN and press Enter. It'll open registry editor. Now goto following keys:
Code:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System
HKEY_USERS\.default\Software\Microsoft\Windows\CurrentVersion\Policies\System
And in right-side pane, look for a DWORD value "DisableTaskMgr". If you find it, delete it. Log off or restart your system and Task Manager should be enabled now.