It sounds like you need this sort of thing:
http://www.amazon.co.uk/Belkin-802-1...3666833&sr=8-1
Although this Belkin model looks as if it's going to need a mains connection, rather than being powered by a USB port.
I might look into getting one myself, my current employer won't allow wireless to be installed in their laptops so I have a 10-metre cable snaking its way out of the dining room to the router located next to our home computer in the next room.