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Bots, or "backdoor Trojans," can turn a computer system into a "zombie," meaning a hacker can send remote commands to it and perform a variety of tasks. This can include the spread of malware and sending spam, or using the network of computers to launch cyber attacks.

Microsoft said in a report that the backdoor trojans "are a significant and tangible threat to Windows users." However, the bots are not the only threat that Windows users face.

Social engineering attacks were found on 35 percent of machines, and rootkits were also an issue on 14 percent of the PCs scanned. Without the Sony rootkit, however, this number falls to eight percent. The company also noted that in one out of every five computers with a rootkit, a bot program was also found.

"Rootkits ... are a potential emerging threat but have not yet reached widespread prevalence," while "social engineering attacks represent a significant source of malware infections," Microsoft said.

Of the 5.7 million infected computers scanned using the tool over the past 15 months, nearly 16 million instances of malicious software were removed. This averages to at least one piece of malware being removed for every 311 computers scanned.

Since the release of the tool in January 2005, the program is now available in 24 languages to users of Windows 2000, XP and Windows Server 2003. It has been run 2.7 billion times on 270 million unique computers.

The tool can detect some 61 different types of malware, Microsoft says. The company added that 41 of those types have seen decreases in detection since the programs launch, with 21 having decreases of greater than 75 percent