CWmike writes "Machines running the decade-old Windows XP make up a huge reservoir of infected PCs that can spread malware to other systems, a Czech antivirus company said. Windows XP computers are infected with rootkits out of proportion to the operating system's market share, according to data released Thursday by Avast Software, which surveyed more than 600,000 Windows PCs. While XP now accounts for about 58% of all Windows systems in use, 74% of the rootkit infections found by Avast were on XP machines. Avast attributed the infection disparity between XP and Windows 7 to a pair of factors: The widespread use of pirated copies of the former and the latter's better security. Vlcek assumed that many of the people running XP SP2, which Microsoft stopped supporting with security patches a year ago, have declined to update to the still-supported SP3 because they are running counterfeits."