Windows Vista's market share growth has slowed since Microsoft released public versions of Vista's successor, Windows 7, according to data published today by web metrics company Net Applications.

For the fourth month in a row, Vista's gains in May were lower than its 12-month average. That stretch coincides with the availability of Windows 7 beta in mid-January, and with the one-and-only "release candidate" issued early last month.

Although Vista has consistently posted increases since it was released - almost exclusively at the expense of the aging Windows XP - the size of those gains has fluctuated. During 2008, for example, Vista grew, on average, by 0.9 of a percentage point each month.

In the four full months since Windows 7 was issued to the public, however, Vista's gains have averaged just 0.5 of a point, a little over half as much.

By Net Applications' data, Windows Vista was the operating system used by 24.4 percent of the machines that connected to one of the 40,000-plus sites it monitors for clients, an increase of 0.45 of a percentage point over the previous month. The months before that, Vista increased 0.48 of a point (April), 0.63 (March) and 0.31 (February).

Windows 7, meanwhile, grabbed 0.42 percent of the market in May, an increase of 0.18 of a percentage point that translated into a month-to-month growth rate of 75 percent. The jump was likely caused by the 4 May delivery of the Windows 7 Release Candidate (RC).

Vista's smaller-than-usual gains in 2009 were at times dramatically down from the increases during the same months last year, illustrating the slowing uptake of the two-year-old operating system. February 2009's increase was off by 67 percent from February 2008, while May 2009's gain was down 36 percent from the same month last year.

The numbers shouldn't be a surprise. Reviews of Windows 7 have generally been positive, and an April poll claimed that corporate beta testers were four times more likely to be "very satisfied" with the beta than were early users of Windows Vista.

At the same time Vista's growth has slowed, so has the pace of Windows XP's decline, hinting that users are tightening their grip on the old-but-still-loved OS. Last month, for example, Windows XP's share dropped by 0.67 of a percentage point, to 61.5 percent, a significantly smaller fall-off than either the 12-month average of 0.88 of a percentage point or XP's loss of 0.95 of a percentage point in May 2008.

But not everything associated with Windows 7 was good news last month. While Net Applications noted the near-doubling of the preview's market share during May, analysis of the month's data showed that uptake progressively slowed as the weeks went by.

The first full week after the May 4 release of the RC (May 11-17), Windows 7's share increased 0.06 of a percentage point over the previous week. The next two weeks, however, posted smaller gains over the prior week. During the week of May 18-24, for instance, Windows 7 grew 0.04 of a percentage point over the week before; during the week of May 25-31, the new OS gained just 0.03 of a percentage point over the week before.

And Windows 7, even in RC form, has been unable to stem the continuing slide of Windows overall. Net Applications pegged the operating system with an 87.75 percent market share for May, down 0.15 percentage point from April's 87.9%.

Apple's Mac OS, on the other hand, grew 0.08 of a percentage point in May, accounting for 9.81 percent of the market. That mark was Apple's best since January's 9.93 percent, and again put the operating system on the road to the magic 10 percent mark. Assuming Mac OS posts gains equal to its 12-month average, it would break the 10 percent bar in early July.