Originally Posted by dakramer
Is there a program or programs that can do this task?
If you and your co-worker share hub (not a proper switch), he would be able to use Wireshark
to capture your network traffic.
The only other way he could capture your traffic is if he set his machine up as a network bridge and your network cable was plugged into his machine.
In either case, I should think that reporting him to your superiors would be the correct course of action. Almost all companies have policies against this type of behavior. If your company does not prohibit these activities, then consult with your IT department. Let them know your co-worker might be capturing your network traffic and ask them to help you solve the problem.