The first and obvious thing is to take your laptop and put it beside the PC that is not seeing the router to check that the signal is ok there, if it isn't try powering your router down by switching off the mains power and pulling the power jack out of the router for about 15 secs to let the capacitors drain, re-plug and switch the router back on.
BT fon is a system that allows participating people to connect to the net using any other participating persons 'BT Home Hub', it's very useful on holiday. (It's only available to BT customers.)
edited to add: It's no good, I just type too slowly to beat you lot.
