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