Windows XP has a firewall. Have you disabled this? You should not have two firewalls running at the same time, as they can interfere with each other.

You could update your Java:

Java SE Downloads - Previous Release - JDK 5

Next, try port-forwarding your router (instructions in link):

Port Forwarding for the BT Voyager 220V

As for your ISP, it's the company that you pay for your internet. I see that you put BT in one of your responses. Give this stuff a try and let us know if you have any success.

Good Luck

Dan