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 today.

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.

XP's share of the infection pie was much larger than Windows 7's, which accounted for only 12% of the malware-plagued machines -- even though the 2009 OS now powers 31% of all Windows PCs.

Full story: Computerworld