Internal WiFi card not getting IP

Hi there,

I am having a problem with my Airport in the way of it not getting a valid IP address.
At first I though this was due to the router. No so as my Sisters MacBook (exactly the same model and version) works fine. Also it will not connect to any WiFi signal including WiFi Hotspots.
I though that maybe it was because I am in a foreign country (I am traveling around Asia at the moment). To check I bought a USB WiFi receiver designed for MAC and for Asia.
Still no joy.
I connect to an access point and it comes up with "Self Assigned IP address".

Please can anyone help me????

Thanks
Fraser