Did you ever try killing all unnecessary processes in the task manager (e.g. firewalls, anything produced by Norton, other applications accessing the network)? Maybe there is some incompatibility with some other software you are running.

Last but not least, you may check whether or not you have a direct internet connection... Some proxies / routers cannot be used with LimeWire because they are configured not to allow outgoing connections to high ports (>1024).