Server2003 - Can see network but cannot ping

I'm a developer not a network guy so please forgive my ignorance :-)

I have a small test network with a Win2K Server DC and 2 XP Pro clients. I
recently added a Win2K3 Server to this network but after install I found
that the Win2k3 Server could not ping one of the XP Pro machines and that
the same XP Pro machine could not ping the Win2K3 Server. The two machines
both appear as part of the network and are able to access resources on each
other but cannot connect using Remote Desktop (which is what I really need
to do here). Both machines can ping the DC and the other XP Pro machine
without difficulty.

I do not have RRAS installed on the Win2K3 Server and the internal firewall
is disabled. I have tried pinging by both machine name and IP address with
no success. When I ping by machine name the IP address is correctly
resolved. Both the Win2K3 Server and the XP Pro machine are using DHCP to
get their IP addresses and I have confirmed that they are on the same
subnet.

All service packs and patches have been applied to all machines. There are
no events in the Event Log that relate to this issue.

I'm not sure how to diagnose this problem any further. Any help would be
much appreciated.

TIA.

- Adam