Sometime you receive the message “Task Manager has been disabled by your administrator” when you press ALT+CTRL+DEL or the option “Task Manager” is disabled. It may possible that Task Manager has been disabled or it may be possible that it has been disabled by any virus or Trojan.
You can get rid of it by following the below steps:
 Edit Registry value:
Windows Registry Editor Version 5.00
Write the above code in notepad and save it in .reg format and then double click on it. It will add the required entry in Registry to enable Task Manager.
– Open Registry (Start -> Run -> regedit)
– Navigate to following path:
– From right-side, delete the value named DisableTaskMgr and close the registry editor.
In most cases, one of the above solution works.
For other solutions, you may refer Microsoft kb 555480