Microsoft's new Windows operating system Longhorn will be so different from its predecessors that users may not like it right away, Microsoft chairman and chief software architect Bill Gates said yesterday.

"Longhorn is a bit scary. We have been willing to change things," Gates said during lunch at Microsoft's annual financial analyst meeting in Redmond.

"Longhorn should drive a whole range of upgrades, but that could be sort of delayed," Gates said. Because of differences with the previous versions of Windows, it could be a year or two after its release before computer users really pick up Longhorn, he said.

Recent Longhorn movies leaked to the Web (now removed) illustrated Microsoft's debt to Apple's research and development department: One movie clearly showed a window featuring an OS X-like genie effect. To make this a little different, Microsoft has made the window ripple in a curtain-closing-like effect across the screen as it disappears.

Gates appeared to distance himself from a commitment the company made at its Windows Engineering Hardware Conference (WinHEC) in May to deliver Longhorn in 2005 – he would not comment on the release date yesterday.

"Longhorn is innovative", he claimed, but "there is a lot of work to be done in terms of what has to go in and what has not," he said. Asked if Microsoft would consider dropping some of the innovations it has planned so the product can come out sooner, Gates said no: "If you split it up, then you delay one of the really great pieces," he said.

"We need a big bang release to drive excitement," Gates said.

Microsoft has been tight-lipped about specific features in Longhorn, but early versions of the product have leaked to the Internet. Major changes in Longhorn are expected to include the graphical user interface and either a new file system or a technology update to the existing file system.

The company is also building Office and server products to accompany Longhorn when it ships. As part of the drive, the company announced plans to recruit 5,000 development staff yesterday in a major increase in its r-&-d spend.

Microsoft promises more details about the operating system release, which analysts have said will be one of the most important Windows launches for Microsoft, in October at the Microsoft Professional Developers Conference to be held in Los Angeles. A beta of Longhorn is planned for next year.

As Microsoft prepares its next-generation OS, it faces a tough patent claim by company InterTrust. InterTrust claims 85 per cent of Microsoft's entire product line infringes its digital security patents, according to Fortune Magazine.

"InterTrust's engineers developed and patented what they say are key inventions in two areas: so-called digital-rights management and trusted systems. The technologies are essential to the digital distribution of copyrighted music and movies, and to maintaining the security of e-commerce in general," the report states.

Joris Evers contributed to this report.