All computers are reporting an IP address conflict? It sounds more like a worm or virus problem rather than an actual conflict, unless your running a dhcp service and something wired is happening with it;
Have you tried scaning a few pc's for virus or trojans? I'd advise you also run a packet sniffer and watch the traffic on the network, see if you can identify any wired behaviour.
Let us know how the network is setup, IP addressing scheme, services running (e.g dhcp, dns e.t.c) and anything else you think could help us help you!
THanks for your reply , CHris!.
Below is the arp result I got when I am having the IP conflict problem.
Many PC are having the same MAC 00-0c-f1-1d-45-01 and even my router too(10.1.1.254)!
I do have a DHCP server but all the IP below are static IP.
I'm still not sure the source of the problem but I know that the problem start when we are started using wireless lan.
And most of the case, the problem MAC address is belonged to notebook with both wireless and wire connection.
Thanks for your reply Jack, your suggestion can slove the problem.
But I cannot do it for 300 machines at the same time.
Also, it 's very annoying and it will cost all the network connection down for a few minutes. And it happen 1 to 2 times a week..