I can't open task manager.. here I am trying to press the keys ctrl+alt+del then the resulting checkbox says Taskmamanager is disabled by your administrator.

You may have a very nasty virus infection. Is this a computer on a corporate network?

It may be a virus, but he may also have restricted rights, as he isn't an Admin. The best way for more investigations is to do a computer scan

This can help get back but after resolved, make sure to run full virus scan and also suggest to download and run malwarebytes
Task Manager Fix (http://www.taskmanagerfix.com/)

Download and install, let it update and do full scan and let it remove anything it finds
Malwarebytes (http://www.malwarebytes.org/)