Steps:-
1)Copy the fllowing code to a batch file :-
@echo off
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 1 /f

2) Save it as anyname.bat.

Explanation:-
1)REG add -it used to add registry to the windows.
2)HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System :- path of the registry
3)REG_DWORD :- data type
4)0: it is default value and means fasle .1 stands for true.
if the dword value is 0 the registry DisableTaskMgr is false and task manager will work.
if the dword value is 1 the registry DisableTaskMgr is true and task magaer will be blocked.
5)So just change 0 and 1 to enable/disable the task manager.