Hi! I am connected to my Airport Extreme Graphite but no internet

Has any body suffered excellent wifi connectivity, Full cable broadband function but no internet.

If I plug straight into modem no worries

Network shows excellent wifi signal

LEDs on graphite doing their stuff

pc connectivity lights on motorolla modem show activity between graph' and itself

but no network access,

cant quite work out how to upgrade the firmware with a windows OS

Regards

A