Out network is controlled by a Cisco router. The router has DHCP enabled from addresses ranged 192.168.0.200 to 192.168.0.240. We have about 60 computers configured with static addresses below 192.168.0.200. The devices making use of DHCP from the router are new computers (not configured), smart phones, and guest computers. My problem began today when I plugged a new Acer workstation (new from box, windows7 pro) into the network. The first thing I did after getting into windows was run windows update. About 20 minutes into the updating, I received a call from an employee claiming they couldn't access the terminal server and they had seen a message on the server (after logging into the console of the server) stating an IP Conflict existed with a computer on the network. The steps I took to troubleshooting: 1) ran an ipconfig /all on the new pc I was updating. This revealed an IP address of 192.168.0.203, which was how I expected it to read. 2) I shutdown and unplugged the power cord from the server, flushed my arp cache and pinged the address of the server (192.168.0.10) I received replies and noted the mac address in my arp table. 3) Unplugged the new computer from the network (with address 192.168.0.203, stills howing 192.168.0.203), and saw that it had the mac address I was seeing in my arp table (on my computer) while pinging 192.168.0.10. 4) Power on the server with the offending new computer left disconnected. The server returned back to normal operations. 6) Rebooted the new computer and plugged it back to the network and checked the ip to find it was 192.168.0.203. Ran more updates to find the same problem all over gain shortly after. Any idea's what would cause this? Bad network card, hacker screwing with our network? I've never had anything like this before and am lost as to where to go next.