Hi Nick,
DirectPlay (which is part of Microsoft's DirectX) uses port 6073 for the FS2004 session. So it's not something you can change, unfortunately.
Most likely there's some other program, or hardware driver, that's using that port, and interfering with DirectPlay. The only thing I can suggest is that you try shutting down all non-essential applications, and then try minimizing your network setup as much as possible. For example, disable all your network cards, disconnect from the internet, disconnect from your router, turn off all firewalls (including XP's), etc. Maybe one of these will help you figure out what's interfering with it. I'd start with disabling the network cards, that's actually helped quite a few people in the past.
Russell