|You can't make your private IP public as it's likely a nonroutable Private class IP.|
If you have a router what you do is create a port forward on the router.
For example, lets say you want to run an ftp server on your XP PC and it's LAN IP is: 192.168.0.101
You would login to the management interface of your router and create a port forward that would forward any traffic on port 21 (ftp) to 192.168.0.101 (the ftp server).
To connect remotely to the ftp server, one would ftp to your external IP address. This is the static IP provided by your ISP.