All traffic can always be sniffed by a person outside, no matter what it is.
Sure, if the person some how manages to become part of the network either through (_wired or wireless_). Then they should be able to capture (sniff) the traffic. But until then, you have to find a way to get on/be a part of the network.
The workstations have a lot of spyware,malware etc... on them they need to be cleaned up. I bet a lot of that stuff is because of it.
I would just uninstall it and block them from accessing www.myspace.com
but this is in a doctors office with only 2 workstations and a notebook. I don't see to go to that 'level of security," "so to speak,"