Task manager problem in my pc

Hello, Now I am using Xp operating system. When I am pressing ALT+CTRL+DELETE error message is shown i.e. “task manager has been disabled by the administrator”. Then I am restarting my computer, no positive result comes out. After that I format my pc and my task manager is enabled but when I am restarting again and same error message is shown again. What is the problem? Please answer
Thank You
