step 1: Buy a Linksys Router
step 2: Enjoy an open NAT
seriously though
-go on your PC and get the gateway and DNS numbers that your pc is using (got to command prompt and type "ipconfig /all")
-go to your router and find out what the DHCP range is.
-go to your xbox 360 and setup a static IP OUTSIDE of the DHCP range, that no other computer is using.
-set the DNS/Gateway, etc. based on the setting you grabbed from your PC
-go into your router settings and setup porforwarding to the Static IP you chose for your Xbox and forward port 88 and 3074 UDP and TCP to that IP.
Done...
If your router doesn't support port forwarding... get something that isn't a Piece of Sh*t
If your DHCP IP range takes the entire IP range leaving you no room for a static IP then set the DHCP range to something smaller (as long as it's big enough to handle the max number of computers you're connecting through that router you should be fine... something tells me it's less than 250

)