A study that has been conducted by the AVAST Virus Lab has identified unpatched and pirated versions of Windows XP as the perfect target for rootkit infection. The study found that rootkits infecting via the MBR were responsible for [more than] 62 percent all rootkit infections. Driver infection made up only 27 percent of infection. The clear leader in rootkit infection is the Alureon (TDL4/TDL3) family, responsible for 74 percent of infections.
For more details: http://www.sectechno.com/2011/08/06/pirated-copies-of-windows-xp-top-rootkit-target/