Microsoft has launched its latest version of Windows, called Vista, with more than 100m computers predicted to be using it worldwide within 12 months.
Microsoft founder Bill Gates called the launch a "big day" that would bring a new digital workstyle and lifestyle.
The new operating system (OS) boasts an improved interface and security tools.
But not all PCs will be able to run Vista - Microsoft recommends machines have at least 512Mb of RAM, a 800Mhz processor and 15Gb of hard disk space.
Microsoft has pledged to continue support for XP users until 2011.
The company launched Vista for business users two months ago. Now Vista has been released to consumers, who can buy four home versions.
There is also a stripped down version of the OS, Vista Starter, which is aimed at customers in developing countries. It will be available in 70 languages and will run on slower and older PCs.
Microsoft chairman Bill Gates, speaking at the global launch of Vista at the British Library in London, said: "We have worked hard to build a new software platform.
"We are so excited to see what people are going to do with it."
But Microsoft could face a backlash from consumers over its pricing plans - with the cost of Vista versions in the US roughly half the price of equivalent versions in the UK.
Prices for the OS in the UK range from about £100 for an upgrade version of the Basic package to £249 for a copy of the upgrade to the Ultimate version of Vista.
In the US prices start from $100 (£52) for an upgrade of Vista Home basic to $249 (£127) for the equivalent Ultimate version.
WHAT IS AN OPERATING SYSTEM?
It is the program which manages the hardware and software resources on a computer.
It also forms a platform on top of which other programs can run.
David Mitchell, the software practice leader at analysts Ovum, said: "In the consumer space there has not been any new release of a Windows operating system for five years.
"Vista has taken a long time to develop so there is quite a lot of pent-up consumer demand for it."
Ovum predicts that 15% of XP machines will be running Vista by the end of 2007.
"Part of the appetite is 'something new for something new's sake'," said Mr Mitchell.
VISTA PC SPECIFICATIONS
DirectX9 capable graphics processor
128Mb graphics memory
40Gb hard drive
"There is almost a fashionable element to it. You could argue that XP isn't broken, so why do some people want something new."
Mr Mitchell predicted that the new graphical interface, the improved desktop search tools and a promise of more robust security would appeal to many users.
But he predicted some consumers could be confused by the minimum specifications for PCs to run Vista and by the different versions on sale.
Users can visit the Microsoft website to check if their hardware will run Vista and some new machines are being labelled Vista Capable or Premier Vista Ready, for those PCs with higher specifications.
VISTA HOME VERSIONS
Home Basic - improved search and security but no Aero interface (pictured)
Home Premium - As above but with Aero, Media Center options, back-up tools, DVD burning software
Vista Ultimate - All home and business features, plus a series of downloadable Ultimate Extras
"There's been an attempt to demystify what the minimum specifications are," said Mr Mitchell.
"Undoubtedly some people - as in any industry - won't read the instructions."
Microsoft's previous operating system, Windows XP, was criticised for having too many security holes and needing constant patching.
Hundreds of fixes were introduced to the machine, including two sizeable improvements called Service Packs.
The firm has pledged that Vista is more robust, but will come under fire if Vista proves to be the popular target of malicious hackers exploiting flaws, said Mr Mitchell.
"It's crucial for corporate reputation and revenue that Vista proves more secure and stable than XP," he said.
Features like Media Center are available in the Premium and Ultimate versions
The new operating system has been criticised for the way it handles digital content, such as paid for movies and downloads.
Vista is able to downgrade the quality of video and audio if the content owners - such as a movie studio or download service - want the PC on which the media is being played to have specific connectors, such as a HDMI port to connect to a monitor.
Derek Wall, a spokesman for the Green Party in the UK, said: "So-called digital rights management (DRM) technology in Vista gives Microsoft the ability to lock you out of your computer.
Microsoft has defended its use of DRM saying it is only acting on the requests of content rights holders.
On Tuesday, Microsoft also releases its new version of Office to consumers.