Microsoft released data from 15 months' worth of scanning computers with its malicious-software removal tool:
The tool has been run approximately 2.7 billion times by at least 270 million unique computers, leading to the removal of 16 million instances of malicious software from 5.7 million unique Windows-based computers over the past 15 months, Microsoft said. Sixty-two percent of those computers had Trojan horse programs on them.
Microsoft found that most of those Trojan programs took the form of bot software, which allows attackers to remotely control the infected machines for use in all sorts of online criminal activities, from knocking Web sites offline to spreading viruses, spam, adware and spyware. Bots in the Rbot, Sdbot, and Gaobot families made up three of the top five slots in terms of number of removals. (There are hundreds of variants of each of those bot programs, and usually several new ones surface each week.)