NAT (Network address translation) is the method whereby an outgoing request is paired with the appropriate in bound request - for example if local address 192.168.1.1 is talking on port 3384 to google the router knows inbound requests on 3384 from google are for the machine on 192.168.1.1 and forwards it accordingly.
You are mixing up NAT and PAT (port address translation). PAT (or NAT Overload) is a subset of NAT.
NAT maps an Internal (private) IP address (e.g. 192.168.0.11) to an External registered, routable, public IP address, assigned in this case by your ISP.
NAT can be configured as a single registered address (the usual situation for a home connection), or as a pool of registered addresses (e.g. corporate environment).
With a single external address, if there is only one internal device (e.g. PC connected directly to broadband modem), there is no requirement for PAT at all - simply a 1-1 internal-external IP address mapping. Port numbers are not involved in the standard NAT process.
Where there are more internal devices then external IP addresses, then PAT becomes involved. PAT dynamically assigns a high port number to each internal address as it is translated to the external address outbound, and maintains this port-mapping in a table, so that it can track the inbound replies and map the response back to the correct internal IP address.
In TCP/IP, the combination of an IP Address and port number is called a Socket, so PAT is sockets-based, while NAT is address-based.
SD
PS - To access Remote Connections in XP, right click My Computer, choose Properties, and open the Remote tab.