Do you run XP SP2 or SP1 on the computer you're having problems with? SP2 reduces simultaneous connections & for some strange reason prevents some people connecting with LW. There's fixes such as both MS & 3rd party patches, but the most effective one I've heard of is uninstalling SP2! lol
You have D-Link. Is it port forwarded to your other computer? Only one computer can port forward a particular port no. Each of the others would require different no.'s But I doubt this is your connection problem it would affect your upload ability.
Do you have the identical firewalls, etc. on your other computer you can connect with. Another thought which probably won't work, but least worth a try is to copy your LW preferences folder from one computer to the other.
Back to the SP2: it was moreso designed for the large networked companies. Not so much for the home user. But people update thinking it's the thing to do b/c M$ gives them a message that they should. SP2 cost business across USA & the world a lot of money in loss of income due to the problems it created. SP2 works fine on some computers but not others. That's windows for you. Thanks Bill Gate$.