The manufacturer has just released statistics related to the Malicious Software Removal Tool (MSRT) from its release in January 2005 until March 2006.
During that time, the MSRT has removed malware from 5.7 million unique PCs. 3.5 million of them (or 62 percent) had at least one "backdoor" Trojan, thus called because they open a back door and give a remote attacker unauthorised access to the computer.
Microsoft compiled a list of the top five most removed malware types. Three of the five (Rbot, Sdbot and Geobot) were "bot" Trojans, a type of Trojan that communicate with the attacker through either IRC or instant messaging once the program is installed on a remote computer.
Aside from backdoors and bots, other types of malware such as rootkits were seen less commonly, Microsoft said.
Over the 15 months of the study, the MSFT has been run around 2.7 billion times, according to the figures. In March 2006 alone, 270 million unique systems used the tool. Since January 2005, malware was found on one in every 311 computers.
The MSFT is automatically run on systems with Windows/Microsoft Update enabled.