Two Windows XP Pro machines.
Windows XP and Windows Me have the built in ability to network over Firewire, so if you plug one machines Firewire port into another's you get a 400Mbps connection between them.
Unfortunately Windows XP will only use that connection if a vanilla Ethernet connection isn't avaliable (ie- I unplug it from the back of the machine) then they'll transfer over the Firewire.
Which leads back to my original question, why does Windows always choose the vanilla Ethernet connection?
The Short answer is this:
Ethernet will always be chosen over Firewire in all current systems because Ethernet is currently the networking standard.
If you want to ensure that you Always and ONLY use Firewire connections for file/print sharing in Windows, turn off (remove) File and Print sharing on your Eithernet Adapter.
You will still be able to access the Internet using it, but all File and Print sharing requests (computer to computer) will be forwarded to the only device that supports file and print sharing (i.e. the firewire port).
Hope that helps,