The Storm Worm Botnet currently infects between one and ten million computers worldwide, which means that it has access to a huge amount of processing power and somewhere between 1 and 10 petabytes of RAM. This apparently makes it one of the most powerful computers in the world, with more computing power than the ten fastest supercomputers in the world combined.
These interesting but admittedly vague and flaky estimates come from computer scientist Peter Gutman. Although you can pick at the numbers quite easily, the guy makes a very interesting point. While projects like Seti@Home can harness a lot of computing power, a virus or worm that doesn't need to ask permission from a user could conceivably be vastly more powerful. Imagine the potential if virus writers found more interesting things to do with those cycles than send spam.
Will the first person to find extraterrestrial signals be an amateur hacker, rather than Seti? Could complex protein folding solutions be found by bored crackers? And would the benevolent act of finding a cure for a genetic illness outweigh the malevolent act of creating the worm that rounded up the processing cycles needed to do it? [Uber Review]