Did you try turning off XP's firewall in the control panel? Usually it shouldn't interfere with LimeWire connecting to the network but you never know...

The easiest way to find out if there is a hardware related issue is to download and install another p2p application (like bearshare, shareaza or whatever you prefer) and run it. If it works it's definitely LimeWire's fault.

This thread may be of some help if you want to configure your router:
http://www.gnutellaforums.com/showth...threadid=23369