I'm wondering what this has to do with FreeBSD, I think it is more an IP problem.
How are you connected to the Internet? I asume that your router is connected to the Internet and that you have both the systems (XP left and BSD right) connected to the router. If that is the case, why did you assign your external IP address to you XP system?

I think your ISP assigns your external address to the router (that is connected to ADSL) and your router should assign a private address (something like 192.168.1.x) to the connected hosts (XP, FreeBSD or whatever), this is probably done by DHCP.

What I think you should do (or try), connect the ADSL router to your ADSL link. Configure the windows box to use dhcp and connect it to the ADSL router. Your box should get an IP address from the router, figure out if everything is working properly (check Internet etc.) if it isn't working correct you can try to look in te configuration of the ADSL router (browser to 192.168.1.1, default admin password 1234).
When everything is working correct, you should be able to start the FreeBSD box and the box will start correctly (that is what I assume).

Again I don't think it is an FreeBSD issue but a network configuration issue.