I worked for a small Optometrist office that had only two computers, and no server. Windows XP makes it easy to share files between to computers as peers without any kind of authentication other than you are part of the same workgroup (which is typically called WORKGROUP). They have a son that would like to come in and play on the computer, and mess around and perhaps delete important files. XP in workgroup mode had a very slight way of blocking his access to company files, but if those files were in a shared folder, he had complete access.
After searching around I found I could increase the security options on files like that of a domain. In Windows Explorer click on Tools > Folder Options and click on the View tab. Scroll to the bottom of the page and uncheck Use simple file sharing (recommended), and click OK. Now you can right click any folder and choose to set sharing and file permissions. Remember to create additional usernames and passwords for people on other machines that will be accessing the data on this computer.
If you combine this tip with my previous blog you have set up for yourself the poor man's domain, and put off buying a server for much longer.