Data released today by Microsoft showed that Windows 7's malware infection rate climbed by more than 30% during the second half of 2010, even as the infection rate of the 10-year-old Windows XP fell by more than 20%.
"Infection rates have jumped [for Windows 7]," admitted Jeff Williams, the principal group program manager with the Microsoft Malware Protection Center (MMPC). "We attribute that to the increased presence of malicious software attacks out there."
For the second half of 2010, 32-bit Windows 7 machines were infected at an average rate of over 4 PCs per 1,000, a 33% increase over the approximately 3-per-1,000 infection rate during the first half of the year.
PCs running the 64-bit version of Windows 7 fared slightly better, with an infection rate of 2.5 per 1,000 during all of 2010.
The infection rates were tabulated from scans conducted by the Malicious Software Removal Tool (MSRT), a free utility updated monthly and pushed to Windows users via Microsoft's update services. MSFT detects and deletes selected malware, including fake antivirus programs, worms, viruses and bot Trojans.
Microsoft presented the newest infection numbers as part of its semi-annual security intelligence report, which it published today. The company normalized the data by comparing an equal number of computers for each edition of Windows.
Windows Vista Service Pack 2 (SP2), the latest edition of the problem- and perception-plagued OS, also experienced a jump in infection rates during the second half of 2010, the report noted.
In the second quarter, Vista SP2 was infected at a rate of about 6 machines per 1,000; that climbed to above 8 per 1,000 in the third quarter before slipping slightly in the fourth quarter.
Windows XP was the only one of Microsoft's three desktop editions to see its infection rate drop last year.
The infection rate of Windows XP SP3 -- the spring 2008 upgrade to the aged edition -- fell from a 2010 high of nearly 18 per 1,000 in the first quarter to just over 14 per 1,000 in the fourth quarter, a 22% drop.
Windows XP's infection rate decline was responsible for the global drop that Microsoft charted last year. According to its data, the infection rate for all Windows machines fell from a high of 10.8 PCs per 1,000 in the first quarter to 8.7 per 1,000 in the fourth quarter.
But as Williams pointed out, Windows 7 and Vista are still much less likely to be compromised by malware than Windows XP. Windows XP SP3 had an average infection rate for all of 2010 of 15.9 machines per 1,000, almost five times Windows 7's and double that of Vista SP2's.
"We're still seeing a decrease [in infection rates] for the newer operating systems," said Williams. "And with the broader adoption of Windows 7, more customers are protected."
Microsoft has said it sold more than 350 million Windows 7 licenses since the edition debuted in October 2009.
Microsoft's latest security intelligence report can be downloaded from the company's Web site.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is email@example.com.