We're using SME 5.0b3 here, and have a similar problem. Occasionally, the local NIC just seems to stop working. I can dial in from a LAN PC to another ISP, and ping the external IP of the SME, but I cannot ping the internal IP. Like Dave, there seems to be no pattern, it just happens sometimes, and a reboot fixes it.
Dave, if you just want to run a few simple services on your XP box, you may be able to use port forwarding through the SME, as others have suggested.
http://myezserver.com/docs/mitel/ipportfw-howto.html is great. Just pick the port and protocol, and a LAN destination to forward them to. But as was stated already, that really has nothing to do with your problem, just something else to think about.
We haven't had any problems with ours lately, and it only happened maybe once every few weeks before, so it wasn't much of a problem then. But if there is a valid reason for this, then finding a fix would be nice...