How do I fix the NAT problen on the xbox 360 if I'm using Windows ICS?

Ok, so, I have my 360 connected to my computer and my computer shares connection to the 360. My computer is a bit obsolete and runs Win XP SP1 and I can't install SP2 cuz the intallation gives me an error when it's almost done. So I can't open windows firewall. BUT I have Norton Antivirus 2006 (doesn't have personal firewall). I already allowed the TCP and UDP ports everywhere I've seen but I still get the strict NAT thingy. Now, why do I want to get an open NAT? Because I wanna host on gears of war. And also because the NAT sometimes blocks my friend's and I can't play matches with 'em. If somebody knows how to fix this, then u freakin rock.