There are sometimes where your windows Task Manager get disabled by some virus, you can re-enable it, or if u wish u can disable it also, to do this follow the steps
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.