Microsoft's long-awaited Windows Vista operating system has been unveiled in the UK.The software giant's boss, Bill Gates, marked Vista's European launch at an event in the British Library in central London.Vista is the next generation operating system which follows on from Windows XP and its launch will be accompanied by the unveiling of a new 2007 Office system.Microsoft describes Windows Vista and 2007 Microsoft Office as "the most thoroughly tested software releases in Microsoft's history".A predicted 100 million computers worldwide will be using Vista, which has an improved interface and security tools, within 12 months.Vista has been in development for five years and Microsoft said it had taken 50 million lines of software code and billions of dollars of investment to replace XP.It was launched in Japan last year and was available to businesses before the European unveiling.New systems on sale from this week are likely to have Vista already included. Otherwise the software can be bought separately.© Independent Television News Limited 2007. All rights reserved.