Watching over PCs on a Network

Hello, does anyone know some good software that will allow a user on a PC to secretly keep track of what another user is doing on another PC on a network? I am really sure that an employee of mine is constantly using the internet instead of working but I cannot prove it. I need a program that secretly watches his machine without him knowing. Is this possible without installing anything on his machine. I would preferably like a program that takes screen captures of his browsing.

Thank you.

 

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