You don't say how large your network is. If it's fairly small in one location, it should be easy enough to track it down by checking the MAC address against your switch MAC address tables. This will tell you which switch and which switch port its connected to. You can find it's MAC address by pinging it and then correlating it by running arp -a in DOS.
As others have suggested, once you find which port its on, disable that port.
I also recommend running NMAP against that IP as someone suggested above to see if it returns information about operating system, open ports, etc. You may even get usernames or profile information. This can be useful.