New York: Microsoft has launched the consumer version of its Windows Vista operating system, the most expensive software programme in the world that took five years and $7 billion to develop.

 The massive programme, which reportedly contains 50 million lines of code, went on sale at midnight Monday in 100 launch events around the world, with one of the earliest launches in Tokyo where customers were said to have lined up outside stores to get their hands on the new system.

Speaking to journalists in New York, Microsoft chief executive Steve Ballmer said he expected Vista to be the most successful launch in the company's history, selling twice as many copies in its first three months as Windows XP and five times as many units as Windows 95.

"By midnight anybody in America can buy an upgrade or a new computer with Windows Vista installed," Ballmer said.

Noting the changes since Microsoft first launched Windows 95 and Office 95 12 years ago, Ballmer said, "Today the PC is the centre of how people control their digital lifestyle."

Vista includes improved graphics, tighter security and integrated computer searches. "We have focused on making the products safer with security, parental controls, and anti-phishing," Ballmer.

But some analysts question whether Vista offers enough improvements to prompt users to upgrade their computers.

The programme is being launched in conjunction with a new productivity suite called Office 2007.

Together the two programmes have accounted for some 90 percent of Microsoft revenues.