Windows TaskManager [Registry key to Enable and Disable]:

Hive: HKEY_CURRENT_USER
Key: Software\Microsoft\Windows\CurrentVersion\Policies\System
Name: DisableTaskMgr
Type: REG_DWORD
Value: 1=Enablethis key, that is DISABLE TaskManager
Value: 0=Disablethis key, that is Don’t Disable, Enable TaskManager

Here is an another Way to do the same :

• Click Start
• Click Run
• Enter gpedit.msc in the Open box and click OK
• In the Group Policy settings window
  • Select User Configuration
  • Select Administrative Templates
  • Select System
  • Select Ctrl+Alt+Delete options
  • Select Remove Task Manager
  • Double-click the Remove Task Manager option

And as I mentioned above, since the policy is Remove Task Manager, by disabling the policy, you are enabling the Task Manager.