IP address conflict on WLAN

We just got a new computer to join the other two on our WLAN and started having trouble with the router throwing people off the network. Today one of the old computers was on at the same time as the new one and it got a message saying it had the same IP address as another computer on the network. The new one was the only other switched on at the time. The new computer has Windows 7. The one it came into conflict with has Vista Business. Do I need to give the new one a different IP address? If so how? How could it have come to have the same IP address as the old one?

Sorry if this is the same as another query. After the previous try i got a message saying ATTEMPt T FLOOd so I don't know whether it went off.

Using DHCP on the network? Do they actually have a duplicate IP when you checked them? If DHCP make sure you don't have two different servers handing out IPs.

And yes you need to change the IP on one of the systems.