I want to spy networked sys?? how to do that?

i have 3 sys in my room and it has connected through LAN..... i want to spy what others doing in their sys while they are working.. i want to overlook their activities from my sys while they are working in their sys.. Pls help me... is there any technologies are software to fix my pbm?? if there pls let me knw where can i download that n how to use it