An interesting analysis by AVAST which illustrates statistics from their cleaning of system infections.  While XP is more widely installed, there is improved protection within WIndows 7 from these types of attacks.

Windows XP – Main target of Rootkit attacks
http://blogs.pcmag.com/securitywatch/2011/07/xp_remains_main_target_of_root.php

QUOTE: Research from AVAST Virus Lab shows that Windows XP remains the main target of rootkit infection. In a six month study analyzing over 630,000 rootkit samples, almost 3 quarters of them originated from Windows XP systems, 17% from Vista and 12% from Windows 7.