Is your system is affected by virus and disabled your task manager?administrator has blocked task manager.Do you have these problems? then try the below trick to enable task manager
1. Go to Start->Run and type gpedit.msc press ok
group policy window will open.
2. In the group policy window navigate to User Configuration->Administrative Templates->System->Ctrl+Alt+Del options
3. Then open Remove Task Manager, in that there are there options Not Configured, Enabled, Disabled, by default Not configured is selected
4. To enable the task manager select disabled and press ok.
5. To disable the task manager select enabled and press ok.