Windows XP's user share plunged to 33.7% of all personal computers in August, a record-setting one-month fall of 3.5 percentage points, according to metrics company Net Applications.

When XP's share of only those PCs that are powered by Windows was calculated, the decline was slightly sharper, from 40.6% of all Windows systems in July to 36.9% in August, a drop of 3.7 percentage points.

However it's measured, XP's plummet was dramatic. The decline easily bested XP's previous record of a one-month slide set in December 2011, the month after "Peak PC," the industry's high-water mark and when Windows 7 was quickly gaining ground at the expense of XP.

XP's loss was made up by other Microsoft operating systems, the one-year-old Windows 8 and the four-year-old Windows 7, with the gains split 2-1 in favour of Windows 8.

Windows 7 grew its user share of Windows PCs to 50% last month, while Windows 8 boosted its share to 8.4%, a record for the struggling operating system.

Microsoft has beaten the dump-XP drum for more than two years. Last month, it did so again when a manager in its security group warned that the aged OS will become a prime target for cyber criminals once security updates end on April 8, 2014.

But those calls by Redmond have gone largely unheeded.

In the 12-month stretch from August 2012 to July 2013, for example, Windows XP lost an average of half a percentage point each month, or one-seventh of what it shed last month alone. More recently, XP's decline had actually slowed: In the six months from February to July 2013, XP fell just four-tenths of a point per month on average, or about one-ninth its August decline.

It's impossible to tell whether the XP slide represents actual abandonment of the OS and replacements of older PCs, since Net Applications measures only online activity. The decline, or part of it, could have been caused by fewer XP owners using the internet, or at least the very small part that Net Applications monitors.

And since Net Applications' methodology relies on weighting its data by country, a small decline in XP usage in China, where more than 70% of all personal computers run the operating system and the internet population is enormous, may have had an outsized impact on the results.

Rival analytics vendor StatCounter, for example, showed no corresponding decline in XP's usage share for August: According to the Irish firm, XP actually gained one-tenth of a percentage point last month.

StatCounter and Net Applications tally shares in different ways. StatCounter counts page views - a metric best described as "usage share" - while Net Applications examines unique visitors, a number often labelled "user share."

Windows XP's huge drop last month made shambles of earlier estimates that forecast it would still account for more than a third of the world's personal computer operating systems at the end of April 2014. After the large decline of last month, revised projections now peg XP's expected April 2014 user share at a lower range, between 23% and 28%, based on the latest three-month and 12-month averages, respectively.

Overall, Windows slipped by four-tenths of a percentage point to 91.2%. Linux, which gained three-tenths of a point to end August with 1.5%, and Apple's OS X, which grew by a tenth of a point to 7.3%, took up the slack.

Within the Windows universe, however, there was plenty of movement, as XP's decline best illustrated.

Windows 7, which has assumed the mantle as the standard in business, boosted its user share by over a percentage point, climbing to 50% of all machines running a Microsoft operating system. Windows Vista continued to lose users, falling to 4.5%.

Most of the share lost by XP, however, ended up in Windows 8's camp: The newest OS grew by a record 2.5 percentage points to close August with 8.4% of all Windows-powered systems.

Windows XP's last public security update is planned for April 8, 2014.