The worst hacks of all time

The largest computer worm in history infected tens of millions of Windows PCs in more than 200 countries. U.K. government PCs were hit particularly hard.

What made Conficker so hard to stop was that it could infect computers and distribute itself to other PCs on the same network automatically -- people didn't have to do anything to make it spread.

Conficker wreaked havoc on the PCs it infected, disabling important system services and security software, automatically downloading files and preventing users from visiting websites that would let them download security updates.