Microsoft announced yesterday that it has begun the two-year end of life countdown for Windows XP and Office 2003.

When Microsoft finally retires Windows XP on April 8, 2014, it will have been the company’s longest-lived operating system at 12 years and 5 months. The second longest supported OS was Windows NT, which had a lifespan of 11 and a half years.

“Windows XP and Office 2003 were great software releases for their time, but the technology environment has shifted,” wrote Stella Chernyak, a Microsoft marketing executive, in a blog post.”Technology continues to evolve and so do people’s needs and expectations. It is in a company’s – and its employees’ – best interest to take advantage of the modern Windows and Office software that is designed with these needs in mind.”

Microsoft has kept both Windows XP and Office 2003 alive for so long because, despite the software’s antiquity, it still has a sizeable user base.

Net Applications, a software market research firm, reports that Windows XP’s market share is dropping, but at a rate which would leave a significant number of PCs using the OS by the time it reaches end of life. As per the firm’s research, Windows XP is projected to have a 17.1% market share in April 2014.

“Our recent Symposium survey [in October 2011] had respondents telling us they’d have 96% of their PCs migrated off XP by end of support,” said Gartner analyst Michael Silver to Computerworld. “But 16.5% of organizations say they will have more than 5% of their users still on XP after support ends.”

As enterprise OS migration plans take between 1-2 years to implement and execute, Microsoft is adamant that transitioning away from XP needs to be done with a sense of urgency.

“If your organization has not started the migration to a modern PC, you are late,” Microsoft said.