Unable to use "shutdown" command on the network

Hi guys,

I am running a small home network of 2 PCs via KCORP KLG-575 Gold
gateway/router. One PC is connected to the router via Ethernet cable and
another one is on Wi-Fi connection. Both PCs are behind NAT and are
allocated dynamic IPs by the router DHCP. I can see all required files on
both PCs and they are both correctly displayed in the appropriate My Network
places folders. Both PCs run Win XP Home edition SP1. By some sort of
reason I can't remotely shutdown or restart any of them using "shutdown"
command from cmd. Diagnostic is: "The network path was not found". I
believe it is the router firewall that blocks the command but I don't know
which port needs to be opened for this command. When I connect both PCs
directly P2P via Ethernet cable and disable firewalls on each machine
everything works fine.

Any ideas?