How do you control any PC remotely without telling the user?

You don't. This is illegal (unless you are the owner, in which case you would be the user).

There are, however, programs for the iphone that you can buy to monitor what your computer is doing.

No I am a Network Engineer and I want to see if any of my user is using other applications which are no the part of their job. so please tell me how to do that?