Is your other LW running at the same time? I wonder if it's stealing all the bandwidth. I'm running out of ideas.

Sometimes a version of LW won't like a particular computer's set up whilst it'll work fine on another. You could try an older version to see if it makes any difference. Or even the Beta. http://www.limewire.com/english/content/beta.shtml or older versions here: http://www9.limewire.com:82/download/