Microsoft collects infection data from several sources, including the Malicious Software Removal Tool (MSRT), a free utility it distributes to all Windows users each month that detects, then deletes selected malware. It then normalizes the data by comparing an equal number of computers for each edition of Windows. The measurements are expressed as X per thousand: Windows XP SP3's infection rate, for instance, was 9.5 in the second quarter, or 9.5 XP SP3 machines out of every 1,000.
The x86 editions of Windows 7 RTM and SP1 came with higher infection rates than the x64 versions, and Windows 7 SP1 was less likely to be infected than RTM. Windows 7 RTM x86 had the highest rate, 5.3, while Windows 7 SP1 x64 had the lowest, just 3.1. But even with that low rate, Windows 7 SP1 x64 had the dubious distinction of sporting the largest year-to-year increase because in the second quarter of 2011, its infection rate was an even lower 1.1.
Microsoft's numbers back up the belief that Windows 7 is a more secure OS than the still-present-in-large-numbers XP, and makes a good case for users of the latter to migrate to the former, a transition Microsoft and industry analysts have long supported.
If history is any guide, Windows 7's infection rate will continue to climb as its market share does the same, and won't begin to decline until a successor replaces it on a large number of PCs.
"There is probably no single technology feature set that can explain infection rates in either incline or decline," said Storms. "It has more to do with what the attackers want to attack. And as we have seen, attackers generally get what they want."
The 146-page Security Intelligence Report Volume 13 can be downloaded from Microsoft's website.
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 email address is firstname.lastname@example.org.
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