While Installing new copy of window or due to any virus attack, sometime, Window has disable Task Manager. If you want to enable or disable task manager manually, do the below steps one by one:
1.    Click Start icon of your window (left side)
2.    Click Run or Press start+R
3.    Type REGEDIT and OK
5.    Browse the following
6.    HKEY_CURRENT_USERSoftwareMicrosoftWindows CurrentVersionPoliciessystem
7.    In the Right side, See the value of Disable TaskMgr
If Value is 1 , that is disable Task Manager
If value is 0, that is enable Task Manager
8.    If is disable then by Right click DisableTaskMgr and select Delete. (When prompted with "Are you sure you want to delete this value", select Yes.
9.    Close the Registry Setting and reboot the system.
10. Now click on the Task Manager