Go into the port forwarding section, and direct that port (i believe it is 26755) to the computers LAN IP Address.
This could be problematic if the computer is set up to use DHCP, which it probably is. The assigned IP address may change.
The better solution depends on whether Win500 uses UPnP. I took a quick look at the web site and couldn't find anything concerning this. If it does, configure the router to allow UPnP (which gives applications control over port forwarding) and you're done. If it doesn't, you should configure your computer to use a static IP address and then do the port forwarding to that address. (How to do this correctly is a conversation I don't have time for at the moment.)
If this one computer is the only one to ever access the router, you'll probably be okay doing just the port forwarding like K8TEK described, but it might stop working at some point. Then you'd have to go back to the router and change the port forwarding.