Make sure that your router allows access to port 28000 at least, I know you said you had a modem, with no mention of a router, but most people have some sort of router or network hub. If you can't get any connection through after we have tried to help you, then contact your ISP for further support.
You may have Nvidia ActiveArmor, which is a hidden application that can seriously screw up your connection, this program usually comes in Nvidia nForce chipset drivers, I say this because it has proven to be a constant pain because it conflicts with BitTorrent.