it maybe security. Run the XBox network test. If the NAT is closed or restricted contact your network admin and ask if they have a special way of registering consoles.
As well as you should do is power cycle modem and your router by unplugging power to both and reconnecting power them again.
Unplug modem and router.
Reset the router to factory settings.
Make sure my NAT is open.
Disabled the firewall on the router.
It is the fire wall it needs to be turned off. do so by going into your router and turn it off. The internal, LAN-IP address is normally set to a default, private number. Linksys routers, for example, use 192.168.1.1 for their internal IP address. D-Link and Netgear routers typically use 192.168.0.1. Some US Robotics routers use 192.168.123.254, and some SMC routers use 192.168.2.1. No matter the brand of router, its default internal IP address should be provided in documentation. Administrators often have the option to change this IP address during router setup. In any case, however, the private LAN-IP address remains fixed once set. It can be viewed from the router's administrative console.
Your pc will still be fine if you stilll want a fire wall get one on your pc see this link for a list of some http://download.cnet.com/windows/firewall-software/?tag=bc