For those who don't know, a malicious botnet is a collection of computers that have been quietly hacked and are connected to the internet waiting for instructions. It's a digital army of sorts, with a general not on the battlefield, but sitting in front of his own master control terminal. These “armies” can be used to create havoc for businesses or governments, or can be rented out mercenary-style for spammers or advertisers, giving them a way to make money without getting caught. Most of the spam email you receive comes from a computer in a family's living room while they are asleep at night. They have no idea the machine has been co-opted for these efforts, and the bad guys like it that way!
If you run Microsoft Windows and used the internet in 2008-2009, chances are decent that your machines was carrying the Conficker worm, which made your machine available to evil-doers around the globe along with more than 10 Million other naively compromised machines.
It's too bad that we don't have as many examples of a Digital Robin Hood, going around hacking into machines and then exploiting them for good. In fact, it could start by upgrading all the machines it infects with the latest versions of Internet Explorer and other software that causes these vulnerabilities in the first place!