what\'s ur windows service pack? If it\'s 2, then you don\'t need to disable firewall. It will ask you to unblock applications that use ports n such. I suggest looking up your defaut gateway, just in case you might have modem firewall. Then type it into your browser. If you don\'t come up to a modem\'s page, then I guess you are connected straight through to your ISP.
Hmm, if nothing of this works then I have another feeling. You might be using a ethernet controller that is faulty. It may work to browse the net and have no problems with speed, but it will block many ports for some reason. I had that with old version of either 3com chip or was it D-link. Doesn\'t matter. You might want to try switching that controller if no other reason is found.