The AVAST Virus Lab has identified un-patched and often pirated versions of Windows XP as the main vector for rootkits infections.

Data from a six-month study catalogued over 630,000 samples and found that 74 per cent of infections originated from Windows XP machines, compared to 17 per cent for Vista and only 12 per cent from Windows 7 machines. While Windows XP may be old, it is still the most common operating system around the globe with 49 per cent of Avast! antivirus users having it on their computers compared to the 38 per cent with Windows 7 and the 13 per cent with Vista.

Przemyslaw Gmerek, the AVAST expert on rootkits and lead researcher said that 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. Because of the way root kits attack and stay concealed, deep in the operation system, rootkits are a perfect weapon for stealing private data.

More recent operating systems like Windows 7 are more resilient to rootkits, but not immune, he said.