Wireless Internet Isn't Working

I have a laptop that has a wireless internet function, but it won't work (hence the title). I have a router connected with my cable internet, and I can locate the signal with my laptop. I can get it to connect except it gets "limited to no connectivity" (meaning I can't get online). It says the cause is that the network didn't assign a network address to the laptop. If I try to repair it, it will try to renew the IP address but won't be able to.

A similar thing happened to my friend's laptop a couple of weeks ago. We were at the mall, where there is wireless internet, and he tried to connect with his laptop and got about the same treatment as I do. I do believe he gets internet on it at home, but it may not be wireless, I don't know.

Can anyone shed any light on this?

--For windows XP--

 Go to Control Panel --> Network Connections --> Right Click on the Wireless connection and go to "Properties". You should see a box with the words "This connection uses the following items". Select Internet Protocol(TCP/IP) and press the properties button. There should be two buttons you can select; "Obtain an IP address automatically" or "Use the following IP address. Select the "Obtain IP address automatically" button and click "ok". You just enabled your DHCP client. DHCP is a protocol  that "grabs" an IP address for you. Do the same thing on your friends laptop to get internet access at the mall.

More info...
http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol