Just go to:

• Click Start
• Click Run
• Enter gpedit.msc in the Open box and click OK
• In the Group Policy settings window
o Select User Configuration
o Select Administrative Templates
o Select System
o Select Ctrl+Alt+Delete options
o Select Remove Task Manager
o Double-click the Remove Task Manager option
o Mark remove to restore the task manager.

if you want a video tutorial then here it is:

http://in.youtube.com/watch?v=0OK3D-hUWNQ


but you should have a good antivirus with you. Else virus will administrate you