Firstly, set up a workgroup by right-clicking My Computer, Properties, Computer Name and change the default workgroup name to something a bit more meaningful, also make the computer name recognisable, e.g. MainPC. The computer should reboot. Now identify the folders/files you want to share and right-click them and select share and read/write access. A little hand should appear under those that are shared.
Repeat this on other PCs, use the same workgroup name but, of course, a different computer name. Note that Vista is slightly different, have a look here click here
To copy files, go to Start, My Network Places (Network in Vista) and the computers/folders/files should appear. Open 2 explorer windows and drag them from one PC to the other.