Microsoft has launched its latest version of Windows, called Vista, with more than 100m computers predicted to be using it worldwide within 12 months.
The new operating system (OS) is on sale in four home versions, boasting 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. In the US prices start from $100 (£52) for an upgrade of Vista Home basic to $249 (£127) for the equivalent Ultimate version.