Well, 1st the pc's have to be on the same subnet. If your networking is screwed up of course the SME won't show up. It looks like the router internal IP and the SME IP are ok, but what about the pc's? We need the pc's IP's!!!
If you've managed to screw up the subnets of the XP boxes the SME will treat them as a different network and will ignore requests from them (unless the IBay is set to "Entire Internet" in it's privliages section.
An assumption I am using here is that you are looking within the local lan, not from the outside world. If you're talking from the LAN side of the router you'll be getting into port forwarding. Get it?