Often the actions of viruses (less often - rash actions of users) lead to the fact that the Windows Task Manager is blocked, not responding to attempts to call it.
Few people want to reinstall the entire system because of this, but this is not required - the ways to return Task Manager are available.
The best way is to replace the blocked standard Task Manager with the more advanced function program Process Explorer ( http://technet.microsoft.com/ru-ru/sysinternals/bb896653 ), Which has a significantly expanded functional.
If you want to return the originalTask manager, or installing another did not help, try using the Windows registry editor to check the [HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionPoliciesSystem] key and, if there is a DisableTaskMgr entry with a value of 1, delete it.
You can enable Task Manager through the toolGroup Policy Management (gpedit.msc). There, in the "Features Ctrl + Alt + Del" section, find the "Delete Task Manager" option and set it to "disconnected".