Networking, ADSL Sharing and Trojan

Our Client have query as follows:
We use ADSL connection between 11 people, each using a
LAN card and cable to the switch, in one building.

The problem I’m always facing is when any of these
connected computers gets infected with a trojan, which
dramatically slows down, not just the infected
computer, but all other connected computers as well.

Does anyone please know how can I protect the network
from such trojans?
All computers have anti viruses installed, ut they do
little to protect against trojan infections. I’ve
recently got ‘Deep Freeze Enterprize’ which is
supposed to protect all computers connected in a LAN
network. But I dont know how to set it up for such
task.