Your Win XP machine, or your router should get he IP address. IT kind of depends on how your ISP has you setup on their network and how your home network is setup. Typically, your ISP provides you a static IP address, that gets assigned to one machine that is the web server. Your ISP is responsible for setting up and storing your Domain records, A Record, MX Record, etc. That way when someone types in the domain name, it routes to your machine with the IP address.
You can wait on hold and check with your ISP. You can also assing that IP address you got to your network card in the TCP/IP proprties. BEWARE! If you fail to do this right, you will loose you internet connection and have to set everything back to the way it is now. So it is wise to write down or take note of how your NIC is setup now.
.:TUCK:.