"Btw, are there any arpwatch programs for windows that monitors the network?"

ettercap for windows (linux port, but seems to be half decent in functionality, not sure on XP anymore, but server's should be fine, not sure how ettercap uses raw sockets in windows), can detect machines running network cards in promisc, and can display who's running mitm attacks (not sure exactly how this works)

other than that, Snort IDS can detect an ARP Spoof and kill the connections, also a port to windows