You need XP Professional or Media Center Edition on the host to remote desktop, Home Edition will not work. So if you have Home Edition on that host PC, you cannot use Remote Desktop..
However, VNC works great as well. For Remote Desktop or VNC, you will either need to enable DMZ on the router and point it to the local ip address that your machine has been assigned, or you will need to forward the ports (3389 - Remote Desktop, 5900 - VNC - Default Ports) to that IP address under port forwarding. Once you have the ports forwarded correctly, you need to get the Public IP Address. This can be done by either going into your router and finding it, or simply by going to
www.ipchicken.com or
www.whatismyip.com. Then you simply need to find another PC and use the RDP or VNC client and connect to it. Keep in mind that if you have both options as your disposel, Remote Desktop is superior to VNC.