Your router will get the ISP's WAN (Public) IP address, and then your individual PCs in your network will be differentiated by addresses in the LAN (Private) IP ranges, typically 192.168.0.x or 10.0.0.x
Hit the windows start button...then run...then "cmd"...then "ipconfig" to establish which machine is allocated which IP.
On my router you can force an IP to a certain MAC address which is handy; otherwise your router may allocate the IPs on a first come first served basis...which if you've only got 2 machines isn't the end of the world.
Anyway; using ZA...on the machine you need to access, just add the LAN IP of the other machine into the trusted zone or whatever it's called *
* This advice comes with no warranty - I usually just fuck about with network settings until something works