You'll need to configure a few things on both pcs and turn file and print sharing on for the computer that has the file you want to xfer. assuming it has Windows XP you can do this by going into the control panel, network connections, right click and choose properties on the 1394 (firewire connection). Select internet protocol (tcp/ip) and change the ip address from configure automatically to manually and enter 192.168.0.1, subnet mask 255.255.255.0. Then the the same for the other pc but use 192.168.0.2 instead.
Then enable file and print sharing on the pc with the file. If it is in the list with the Internet Protocol (TCP/IP) check the box if it's not. If it's not listed there click install and choose it from the list.
Once that's done you need to share the file/folder. Right click the folder the file is in and choose share.
Now, from the otehr computer click start-run and type \\othercomputername The share should then be listed. Open it, find your file and copy and paste.