well if firewall is stopped thats ok on linux, since you have a "firewall" in your router
about the other thing, do your linux and windows maschine (when they are on the SAME pc) have the SAME ip address?
if yes it should work
if they are seperate maschines anyway, or have different IP address, make sure you change the default ports in AMULE to something differnt, since you cannot forward the SAME ports to TWO different IP address.
and btw in some routers its called NAT in some port-forward, in teh next its virtuell server, but it all means the same thing, so dont worry you should be in the right sectionof the config menu
stefanero