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Computerworld - Windows XP may be the last Microsoft operating system to hold a majority share, according to data from Web metrics company Net Applications.

At its peak, Windows XP powered more than eight out of every 10 computers worldwide. In November 2007, the earliest month for which Computerworld has Net Applications' data, XP accounted for 83.6% of all operating systems.

Although Windows XP's share has gradually declined since then, it still enjoys a slight majority: Last month, XP owned a 52.4% share.

The long lag time between Windows XP's release in 2001 and Vista's debut in late 2006 -- combined with Vista's stumble in the marketplace -- are the factors usually cited to explain XP's large-scale and long-term dominance.

Both conditions could be reproduced -- Microsoft might face future failures like Vista -- but the one least likely to happen again is the five-year gap between upgrades.

That's because Microsoft has taken to a three-year development cycle. Even though the company has been mum about a ship date for Windows 8, the operating system it showed off last week, most experts have pegged its release to the fall of 2012, or three years after Windows 7's launch.

With a regular cadence between upgrades, it's unlikely that any one edition of Windows will be able to duplicate XP's supremacy.

Forecasts based on Net Applications' numbers bear that out.

Using Windows 7's average increase over the last three months as a guide, Computerworld projects that the OS will top out with a 41% share in the third quarter of 2012.

After that, Windows 7's usage share will begin to decline as customers upgrade to Windows 8 or start buying new PCs with the OS pre-installed. That's what happened to Vista. The problem-plagued OS peaked at 19% in October 2009, the same month that Windows 7 launched. Since then, it has lost nearly half its share; in May Vista accounted for 10% of all operating systems.

By the third quarter of 2012, XP's share will have dropped to 38%, marking the first time the creaky OS will be behind Windows 7 in Net Applications' rankings. Meanwhile, Vista will have nearly vanished, with a share of just 4%.

In this scenario, Microsoft continually repeats the Windows 7 performance, where a new edition is rapidly adopted, comes near but doesn't cross the 50 percent bar, and then is supplanted by a newer version.