Hello all,
I can assure you this is an operating system config issue.
As I have 7 machines on my home network and dozens at work. I will only refer to my home experience in that 5 of my PCs run Win2k and have no problem connecting to the Gnutella clients and the 2 I have running XPSP2, 1 with Home Edition and 1 with Media Center Edition, will not connect. All of which are on a broadband router with a NAT. Putting the XP machines through the DMZ and XP firewall turned off didn't help.
The research I have done so far indicates the issue is focused around the tcpip driver and the number of connections allowed. appearently tcpip in SP2 is limits internet connections (ports or port range).
Trusty Files is a P2P that has a fix for their program but doesn't work for Gnuc.
See: http://trustyfiles.com/help-sw-xpsp2.php
I 'll figure this out soon and post again
Hope I gave a little help to point every one in the right direction.
good luck.