Access to Windows Task Manager on public computers is considered to be a security threat which makes many system administrators to disable it and thus restrict its access. This way all the major critical system resources can be kept under safety.
So, here is the step-by-step procedure of how it can be done on you PC.
1. Go to Start –>Run… (or use the keyboard shortcut ‘Win key+r’) and type regedit and press Ok.
This takes you to the registry editor window.
2. From the left pane navigate to the registry key
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionPoliciesSystem
Now, on the right pane right click and choose ” New->DWORD value ” and name it as DisableTaskMgr and press enter. 
Note: If you cannot find System Key under Policies, create it by right clicking on Policies and choosing New -> Key and naming it as System.
3. Edit the DWORD value by double clicking it and changing the value to 1 with hexadecimal mode selected and press OK.
4. Exit registry editor and Restart your PC.
That’s it! Now Task Manager is disabled and thus inaccessible.
Again if you want to re-enable Window Task Manager, just delete the New DWORD value just created or modify the value data to 0 from 1 and press OK.
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Windows Task Manager also sometimes gets inaccessible due to virus codes. Actually what the virus code automatically does is what we have done above. So next time, when your Windows Task Manager gets disabled due to virus, you can follow the above procedure to re-enable it and thus clean the virus.