Nearly three quarters (74 percent) of all rootkit infections originate on a PC running Windows XP, says Avast.

Research by the security firm's Virus Lab revealed that almost half (49 percent) of Avast antirvirus users run the aging operating system, while 38 percent plump for Microsoft's latest iteration of the platform, Windows 7, and the remaining 13 percent use Windows Vista. However, just 12 percent of all rootkit infections originate from Windows 7 machines, and the remainder from PCs running Vista.

"One issue with Windows XP is the high number of pirated versions, especially as users are often unable to properly update them because the software can't be validated by the Microsoft update," said Przemyslaw Gmerek, Avast's lead researcher.

"Because of the way they attack – and stay concealed – deep in the operation system, rootkits are a perfect weapon for stealing private data."

Gmerek said more recent operating systems like Windows 7 are more resilient to rootkits but warned users not to get complacent as they are not immune to infection.

Nearly two thirds (62 percent) of all rootkit infections were targeting the Master Boot Record (MBR) while 27 percent were driver infections.

Gmerek advised web users to keep antivirus software installed and updated on their machines, regardless of which operating system them use. Furthermore, if they suspect there's an issue, they should scan their computers a rootkit removal tool.

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