It's easy in Windows XP, and I have done it a few times using the FAST Wizard to transfer profiles from one computer to another.
Connect the two using the appropriate cable (4-4, 6-4, 6-6, whatever.) Go into your network properties and repair the 1394 network connection on both computers. Each will attempt to renew the IP address then fail, assigning each one an autoconfigured IP address on the same subnet.
Now you have created a Firewire IP network between two Windows XP computers and can do all of the things (like shares) you can do on an Ethernet network. Anything beyond the physical connection is an exercise for the reader.