Do you know the brand & model no. of your cable modem? Are you using XP SP1 or SP2? Do this simple test for us. Using your browser go here:
http://www3.limewire.com:6346/ The page should say ‘Test Page was a Success’. What happens when you go there?
So far you've done everything right so let's hope it's not your ISP. May I ask who our ISP is?