|"XP to remotely access any pc across the WAN"|
Since RDP uses port 3389 on the lan you would only have to specify the host like jims computer or 192.168.1.12 without a port number.
To go across a router [WAN link] you have to do two things
1. forward the RDP port in the router
2. change the listening port on the distination pc for RDP
This means pc1= port 3389, pc2=port 3390, pc3=port3391
To access via the wan you have to specify what ports are forwarded to which internal ip. For example:
wan port ip: port number for pc1
wan port ip: port number for pc2. etc.
How are you accessing pcs across the wan? What are you putting in for the RD config?