Many ISPs shape the bandwidth of p2p users.

http://www.gnutellaforums.com/202017-post38.html
::. Kitz - ISP Caps, FUPs and Prices .::

So...if you can't get it to work at all, you may want to switch your ISP. First, you could try port-forwarding your router. Select your model from this list. Then click on Limewire in the next list.

PortForward.com - Free Help Setting up Your Router or Firewall

Good Luck

Dan