Well first off, are you sure you have TCP/IP setup properly (if you're using it, if not try NetBEUI). Next would be to try and ping the loopback address which is 127.0.0.1 on each machine and see if that responds. If that works try pinging the actual IP address on the machine. Asumming all this checks out, try pinging the other machine, if that works then everything is fine, if not you may want to try switching the ports on the hub, and maybe even a different set of CAT5 cables.
BoNeZ