Windows 7 reached the 10% usage milestone almost a year faster than its berated predecessor, Vista, did, according to Web analytics company Net Applications.

But the growth of Windows 7 has yet to have an impact on Microsoft Corp.'s overall share of the operating system market, which returned to its usual downward trend last month after a one-month advance. Windows dropped to a 91.6% share, down half a percentage point from February.

The new operating system again grabbed share from both Windows XP and Vista, with the former losing twice as much, on a percentage basis, as the latter. Windows XP slid to 64.5%, down a full point, while Vista lost one-half of a percentage point to end at 16%.

Vista appears to be on a fast slide to nowhere: March was the fifth consecutive month in which the beleaguered version of Windows lost share, and it was the sixth month of the last seven in which it did so. If Vista maintains its trajectory from the last three months, it will drop under 10% before the end of this year.

Windows 7's and Vista's paths should cross in June, when Windows 7 is projected to become Microsoft's second-most-used operating system.

Windows XP still accounts for the majority of the editions of Windows in use. More than 70% of the computers using Windows in March were running XP, which has three years of life left before Microsoft pulls the support plug in mid-April 2014.

Windows 7's gains show some signs of slowing, according to Aliso Viejo, Calif.-based Net Applications. March's 1.5-point increase was slightly off the new operating system's monthly average gain of 1.7 percentage points during the period from November 2009 to February 2010.

Even so, Windows 7 beat Vista to the 10% mark by nearly a year. While Windows 7 reached the milestone just five months after its public release, Vista didn't hit that mark until May 2008, 16 months after its retail debut.

Meanwhile, Mac OS X's share of the operating system market rebounded by three-tenths of a percentage point last month, with Apple Inc.'s operating system ending the month with a 5.3% share. The increase countered a recent downward trend in which Mac OS X lost share in three out of the four previous months. March's increase was the largest for Mac OS X since December 2008.

But for Vince Vizzaccaro, executive vice president of marketing at Net Applications, the news last month wasn't the 10% for Windows 7 but the continued increase in the use of mobile devices to go online. By Net Applications' numbers, mobile operating systems powered 1.7% of the hardware used to access the Web; that's an increase of 0.15 of a percentage point, and double the mobile share of August 2009. "Mobile growth is now more a curve than a straight line," observed Vizzaccaro.

Net Applications measures operating system use by analyzing the pool of about 160 million unique visitors each month to its clients' sites. March's operating system data is available on the NetApplications.com 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 gkeizer@ix.netcom.com.