The files that make up the Intranet will most likely reside on a server, not on an individual's PC. Therefore, you will need to set up the server properly to handle the Intranet.
There are three ways the Intranet server can be set up. The easiest is just to make a map to a network folder. If you do this, FrontPage extensions will not function properly and the Intranet will just be accessing files from a central computer. This is fine for smaller companies, because the server will not have much traffic.
To set this up, you can simply create a mapped drive on all of the computers that will be accessing the Intranet, and you can then simply enter: //servername/drive/index.htm to access the Intranet. When you want to upload files to the Intranet, simply put the files into the folder that you mapped to.
If you have a medium size company and the Intranet will be receiving a bit more traffic, you can install a server on the server computer called Microsoft Personal Web Server. It is made for smaller networks. To access the Intranet, you can type in http://servername and then follow the links to the pages like a normal Internet web site. This is fairly easy to set up because a wizard guides you through the whole setup process. You can search for documentation and downloads for the Personal Web Server at Microsoft's Web Site.
For a larger network, you can set up Microsoft Internet Information Server. This has a much more complicated setup and publishing process, so you can read about it on Microsoft's Web Site.