Thanks for the reply.
I don't have both of the machines on the network at the same time. I connect one (WinXP) and it works. Disconnect that machine and then connect the second (WinXP MCE) and it doesn't work.

I tried the port forwarding on the router for all ports in the range 6000 - 7000 and it still works the same way.

If it works on one machine and not the other, is it safe to rule out any network connectivity issues?

Thanks in advance!