Wireless connection with invalid IP

Server 2003 network, Cisco wireless, W7 laptops.

We have a network with wireless access points for about 20 laptops. A number of the laptops have an issue connecting to the wireless after they've been connected elsewhere. They will connect with a limited connection, with an error message indicating they have an invalid IP. ipconfig /all reveals that they have an automatic private IP, and they won't give it up for the wireless.

If I hardwire the laptop to the network for a minute, it will pick up a network IP, and then I can disconnect it and it will work via the wireless. However, once the user has connected to another wireless connection, the problem returns.

Any ideas are appreciated.

Thanks.