Grasping at straws now - Any firewalls? Client/router sided? have a look around your router; they can sometimes cause problems. Ninja - You may want to try putting your router firewall into "snooze" temporarily. You could also try setting your machine as the DMZ.Looked at your router's manual...It seems you have quite a few choices on the UPnP settings; I would recommend....Enable full Internet Gateway Device (IGD) support and nothing else.
Just plug your CPU into your modem(where the internet comes into your house).
Ethan - Did you try disabling the router's firewall?