Do you have a Modem/router type device to connect to the internet? It sounds like you are connecting to the configuration interface for this.
Alternatively: On what platform do you use XAMPP? Could you post the output of ipconfig in Windows or sudo ifconfig in Linux/MacOS?
OK, so with the info you gave here, it is now clear what the problem is (and that this belongs on superuser.com instead of Serverfault).
When you try to connect to your public IP address, you are in fact reaching your router, not your XP box. If you really would want to connect to the XAMPP server with this public IP address, you would need to configure your router to forward port 80 to your XP machine so that it is accessible from the outside (you will find how that is done in the routers documentation). However, this is a terrible idea if you just want to test things and don't need others to access it. Instead, you would either use
127.0.0.1, which would only work on your XP box, or
192.168.254.4, which would be accessible from any computer in your home LAN.
Could you edit your original question and tell us why you want to use the IP address instead of
http://localhostin the first place?
NB: The info you gave on the router/modem stated that it's firewall is deactivated. This is normally a really bad idea, I would strongly recommend turning it on. How this is done will also be explained in the documentation.