Limited Connectivity / Does not have a valid IP configuration

I've been using my netbook to get online for some time. However, recently my dad needed to reboot our modem several times because he was encountering problems with his laptop. This got the wireless working for a little while, but the need came to restart the modem again, so he did. Unfortunately, this put the wireless on the modem out of action. Originally, my laptop wouldn't even recognize the network's existence, but after a 3-hour-long tech support call to AT&T, our internet service provider, I finally got my laptop to recognize the network. Although the tech support lady was very helpful, and walked me through doing numerous things to both the laptop and modem, my laptop still will not connect fully to the network. By "will not connect fully" I mean that it gives me an error message, saying "connection unsuccessful", and on the list of networks, listed next to my network it says "Limited Access". I've attempted to troubleshoot the problem multiple times, and so far the only thing that comes up is the message "Windows cannot connect to [name of my network]", and something saying I don't have a valid IP configuration. I have full access to the modem's configuration as well, and I've checked the firmware and other things to see if there is a problem with the modem. After numerous tests and diagnostics, nothing seems to come up that says there is anything wrong with the modem, but the limited access problem still occurs for my Dad's, my Sister's, and my laptop. Wired connections seem to work fine though. Can anyone please help?

I'm not absolutely sure but I think I might have an idea as to the problem. I'm assuming your modem also functions as a router being you mentioned you connect to it through wireless, this being the case it must also function as the DHCP server for your network. If the computers attempting to connect to the network are not set up to use DHCP, or the modem itself is not set up to act as a DHCP server, that could be causing the IP configuration error.

DHCP seems to be set up. Any other ideas?

I know you said you restarted the modem but have you tried completely resetting it to the factory default settings? It's not the greatest thing to do, especially on ISP distributed hardware, but I've fixed countless networking issues that way.

Yeah, tried that too. Nothing seems to fix the problem! So frustrating!

So you're all connecting via a modem that has wireless router capabilities?

And all PC's connect properly to the internet when wired to it? A none of them can via wireless?

Sounds like wireless properties on the modem were changed. The modem has to be set up such that it's operating in a mode that accomodates all the PC's (802.11a/b/g/n mixed, etc.) and that the wireless security is set where it can accomodate the capabilities of each PC (WEP, WPA, WPA2, etc.). For security it's best to use WPA2 and make sure each PC is properly updated to work with it.

If the modem is handing out IP addresses via it's DHCP server you have to be sure each PC's wireless adapter is set to automatically obtain and address from the modem. The wired adapter settings on each PC are separate from the wireless adapter so since wired works you need to ensure the wireless adapter on each machine is also set up properly.