The number of Windows 7 SP1 and Windows XP machines infected by malware is on the increase, while the number of infected Windows Vista SP2 machines has declined sharply.

Those findings come from the latest Microsoft Security Intelligence Report (volume 13), released Tuesday, which reviews threat prevalence and infection rates seen in the first half of 2012.

According to the report, the average number of infected Windows 7 SP1 machines increased by 23% on 32-bit systems and 7% on 64-bit systems, comparing the last quarter of 2011 to the first half of 2012. In the same time period, the average number of malware-infected Windows XP SP3 PCs increased by about 10%, while the number of malware-infected Windows Vista SP2 PCs plummeted by 33% for 32-bit systems, and 43% for 64-bit systems.

Despite the changing infection profiles, 32-bit Windows XP SP3 machines are now two to three times more likely to be infected by malware than 32-bit Windows Vista SP2 machines, which have the lowest infection rate of any Microsoft operating system, followed closely by Windows 7 SP1 and Windows 7 RTM.

Meanwhile, the report found that "the infection rate for Windows XP SP3 increased" in the first half of 2012 "after declining for several quarters," largely thanks to Dorkbot worm infections, as well as a Trojan downloader called Pluzoks, which is prevalent in South Korea, where Windows XP remains the most-used operating system.