Although enterprises are in the midst of migrating more machines to Microsoft's Windows 7, the aged Windows XP still accounts for nearly 6-in-10 PCs in corporations, according to a recent report by research firm Forrester.

Windows 7 powered nearly 21% of all business PCs used to reach Forrester's Web site in March, the most recent month for which the firm has data.

While that's more than double the 9.5% logged by Windows 7 a year before, the 10-year-old Windows XP remains the most widely-used enterprise operating system by a wide margin: In March, systems running XP accounted for 59.9% of the 400,000 machines that visited Forrester.com.

Ben Gray, a Forrester analyst who co-authored the report on operating system and browser trends, called Windows 7's adoption "accelerating," but at the same time noted that XP retains a majority.

Windows XP is slated to exit all support -- meaning Microsoft will stop providing security updates, aka patches, for the OS -- in April 2014.

Gray was confident that the bulk of XP systems would be retired by then, replaced by new machines driven by Windows 7. "The majority of firms will move to Windows 7 during the next year as the extended support phase for Windows XP approaches," Gray wrote in the report.

According to Forrester, both XP and Vista lost enterprise share to Windows 7 in the last 12 months, but Vista was particularly hard hit, dropping from 11.3% in April 2010 to just 6.2% in March 2011. In Forrester's tracking, Vista peaked at 14% in November 2009, a month after Windows 7's debut.

"It's no surprise, but firms are abandoning Windows Vista in favor of Windows 7," said Gray.

Vista was aggressively criticized by users shortly after its January 2007 release, and panned by most reviewers and analysts.

The only bright lining in the Vista cloud, Gray added, is that because it and Windows 7 share the same code base, companies that did commit to Vista have found smooth sailing when they upgrade to Windows 7.

Microsoft retains a hegemony in the enterprise -- 87.6% of all corporate computers run one of its operating systems -- but Gray said that some companies were bending to pressure from workers who wanted to use their own machines. The trend has been dubbed by analysts as the "consumerization" of the enterprise.

Tablets, particularly Apple's iPad, have been at the forefront of the consumerization movement, but by Forrester's numbers, the Mac has also benefited, with more than 1-in-10 workers now using a Mac.

From April 2010 to March 2011, Mac OS X climbed from 9.1% to 11%, said Forrester.

"Empowered workers attracted to BYO [bring your own] device programs are quickly coming to expect Mac and iOS support," Gray wrote.

Forrester's operating system usage data in the enterprise shows the same trends as other analysis, including that by Web metrics company Net Applications, which tracks global operating use.

Net Applications pegged Windows XP's usage share in March 2011 at 54.4% and Windows 7's at 24.2%, the former slightly lower than Forrester's, the latter slightly higher. (By May 2011, XP's share had fallen to 52.4%, while Windows 7's had climbed to 25.9%.)

Forrester analyzed the operating systems of 400,000 client computers from 2,500 companies that surfed to its Web site in the last year to come up with its data.

The numbers point to problems that corporate IT staffs must solve, said Gray. "Even with Windows 7 as the dominant corporate OS, IT managers will continue to be challenged by increased device and OS diversity," he wrote.

Windows XP and Vista use in the enterprise have declined in the last 12 months, while Windows 7 and Mac OS X have gained ground. (Data: Forrester Research.)

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 gkeizer@computerworld.com.