1. Make sure you have made exceptions on your Vista firewall to allow Gnucleus & Gnutella and port 6346 if that's the port Gnucleus is set to use. If you have a 2nd software firewall then choose between one or the other. Two software firewalls gives you less security not more, and they usually clash with each other giving unreliable results.

2. If you are using a router or modem with a NAT firwall, then make sure the port Gnucleus is using is clear through the NAT firewall. This may require port forwarding. And to use port forwarding you must begin by setting up a static ip. PortForward.com - Free Help Setting up Your Router or Firewall

BTW if you wished to try an alternative, I would HIGHLY recommend Phex to see if you have the same problems.