I saw a program, towards its end, on BBC last night (1-Aug-2004) at around 8.20pm. It was talking and showing about how some British scientists were working on and had already perfected technology to bring all computers down - almost in the whole of Europe - at the flick of a switch.
It was called EMP (didn't catch what it meant) or some such and when used, could simply knock out a computer from every being operational again. Not probably rocket science given that everything in a computer is mostly electronic/electrical and that some amount of purposefully directed electomagnetic radiation could cause quite a significant amount of influence on it's operation.
However the thought that someone was working on it actively and had it as part of the country's national defense strategy (or so to speak) was quite alarming :)