Right now in my fire station we have 6 pcs connected to a switch and route with a peer to peer network (Mainly just for file and printer sharing and network access)

I do not have any of the computers setup with any passwords at all. Lately I have found that there have been times I had to provide wireless access for visitors to our building with laptops by providing them with our network key but then I noticed they were able to access our network files.

I am guessing that my main solution to this problem is to have usernames setup with permissions and everything. But since I am running a peer to peer network I am assuming these usernames and passwords would have to be manually setup on all PCs. Mostly all employees use all of the PCs in the station but I want to restrict access to specific files and folders while sometimes giving access to all personnel for specific folders but not guests.

I am guessing our best course of action is with a server but with budget restraints we are unable to make such a purchase.

Can anyone give me any guidance on where I can start and what my best course of action is? All of our computers use Vista except 2 which are using XP.