Security on Peer-to-Peer Network

I am an IT Admin for a land contract company. All the landmen I support have their own laptops and some connect to the network via wireless on others via LAN. We are on a Peer-to-Peer network and the company is not ready to install a server at our site (BTW…the company has about 5 or so sites in the area.
Each individual landman owns their own laptop. Besides, firewall and anti-virus/anti-spam is there any other security measures that can be recommended? Thanks for the advice in advance.