wireless n modem router issue with windows 7

I have just upgraded my home network to wireless n with a tp-link td w 8960n modem/router. It is cable connected to a desktop running windows xp. set up easily and connects to the internet no problem. i have a wifi laptop running windowns vista and it connects to the network and internet no problem. I also have a wifi laptop running windows 7 and this will connect to the network but not to the internet. i ran the windows troubleshooter and it says it doesnt have a valid IP configuration. IP configuration was set to detect automatically, as was the other laptop. I tried manually configuring but I still cant access the internet on the windows 7 machine. Any ideas? :confused