Well, that looks like it should be fine, i think that means it is listening on all addresses, so if you turn the firewall off then you should be able to establish a connection right enough. So, you might want to check nsc.ini and confirm
;# ALLOWED HOST ADDRESSES
; This is a comma-delimited list of IP address of hosts that are allowed to talk to NSClient deamon.
; If you leave this blank the global version will be used instead.
;allowed_hosts= [/blockquote]is correct...
Also, maybe look at specifically binding the nsclient service to a particular interfaces IP address by configuring** bind_to_address=** which is just below that.
I assume that normal connectivity is fine between your nagios server and your w2k server, i.e. ping works fine, and there's no intermediate routing or firewall problems or anything, and that the w2k server knows the route back to the nagios server? Although to be honest, connection refused would seem to indicate the traffic* is* getting there OK and being rejected rather than getting lost en route. Also, set up the log file and see if anything pertinent appears in there...
Another thing you could try would be to change nsclient to another port, that might help. I once had a server where nsclient had crashed and held the port open and even though it would seem to 'restart' and run ok, nothing worked. I couldn't reboot the thing so I just moved the service to 12490 instead and reconfigured the checks and it was all fine after that...
NSClient is great when it works, but a right pain when it doesn't.