Microsoft has begun counting down the last 1000 days of support remainining for aging stalwart Windows XP as it urges companies to upgrade.

The software giant would stop supplying security patches and hotfixes for all versions of 11-year-old operating system on April 8, 2014, potentially making it vulnerable to issues that may arise after that time.

The day would mark an unusually long support period for a MIcrosoft operating system, lengthened by companies hesitating to upgrade away from XP. Many have only now begun migrations to Windows 7, refusing over past years to switch to Windows Vista.

"Windows XP had an amazing run and millions of PC users are grateful for it. But it’s time to move on," Microsoft's senior community manager for Windows, Stephen Rose, wrote Monday.

"Bottom line, PC's [sic] running Windows XP will be vulnerable to security threats."

Microsoft first announced last year that it would end support for the operating system by 2014, coinciding with a move to wind down supply of machines pre-installed with XP.

Despite continue take-up of Windows 7, however, Gartner analysts last year warned that most organisations running XP would not make the April 2014 deadline, predicting a rush to gain Windows 7 skills.

To ease transition, Microsoft had already flagged that those companies upgrading to Windows 7 would be able to downgrade to XP any time until 2020.

Not surprisingly Microsoft is encouraging users to upgrade to a newer OS, namely Windows 7, which chief executive Steve Ballmer yesterday touted as the on-ramp to its multi-device successor, Windows 8 due next year.

Microsoft has sold 400 million Widows 7 licenses to date, Ballmer said, and ran on 27.13 percent of the world's computers according to June figures from NetApplications.

Despite Windows 7's increased share, Windows XP still ran on 51.13 percent of the world's PCs, according to the analyst firm.