Yes there is a way, but you need to sign up for a free account as a developer at Apple:
http://connect.apple.com
Once you've done that, download the IP over FireWire preview. Install it (on the mac) and you'll get an additional option in Network settings that allow you to assign an IP to your FireWire port. Do the same on the XP box (which has all the necessary drivers/software pre-loaded) and off you go!
More specifically, you need to choose a fixed IP for the firewire port, for example 192.168.123.1 (on the mac) and 192.168.123.2 (on the XP box). Then, in the 'router' or 'gateway' field, put the IP of the other machine (192.168.123.2 in the OS X network settings and 192.168.123.1 in the XP settings). In the 'subnet' field, you have to input 255.255.255.0 on both machines.
Once you've done that, you can see whether the two boxes see eachother by connecting a firewire cable between them and opening up the terminal in OS X and typing:
ping 192.168.123.2
If it reports a 0% packet loss, then you're good to go. The only thing left to do at that point is to share a folder on the XP box and connect to it through the finder in OS X (cmd-K).
Try that first and report back if you need more info...