How to enable Task Manager?

Click Start goto Run, type gpedit.msc and click OK. Navigate to this branch:
User Configuration / Administrative Templates / System / Ctrl+Alt+Delete Options / Remove Task Manager
Double-click the Remove Task Manager option. Set the policy to Not Configured

Use regedit to open the Registry Editor and locate the following key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System.Find the DWORD value named Disable TaskMgr and set it to 0 or delete it altogether

This will make the Task Manager available again, although you might need to log off for the changes to take effect.

This same process can be used to disable the Task Manager, but you might have to create the registry key System under the Policies key. After creating the System key, create a new REG_DWORD (DWORD value) named Disable TaskMgr and set the data value to 1.