Firstly, on the XP machine, set up a workgroup 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 (or you can drag the files to Shared Docs). A little hand should appear under those that are shared.
WinXP Pro is slightly different click here
As you are sharing with a Vista machine, you may need to install the Link Layer Topology Discovery (LLTD) Responder on the XP machine click here
In Vista, right-click Computer > Properties > Change Settings then change Computer Name and Workgroup Name. WinVista is slightly different click here
Repeat this on all PCs, use the same workgroup name but, of course, a different computer name.
Windows firewall is normally set to allow file and printer sharing but if you have a third-party firewall make sure that it allows access so that each machine can talk to the other(s).
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.
Share a printer, here click here