Nearly half of all PCs bought by businesses in 2010 will be running Windows 7, says IDC.

Forecasts by the analysts reveal that 58 million copies of the new Microsoft OS will be purchased in 2010, that's 49.5 per cent of all Windows operating systems expected to be sold. IDC says the numbers dwarf Vista, which will account for 15 per cent of the 2010 Windows operating systems Microsoft ships that year - that's 18 million units.

To characterise how fast Vista will fade, Windows XP, the Vista predecessor, will account for nearly 35 per cent, or nearly 41 million units, in 2010 even though the OS, which shipped at the end of 2001, will be a year into its extended support phase.

In 2011, Windows 7 will jump to nearly 75 per cent of units shipped while Vista will slip to 0.5 per cent.

IDC says that Vista will drop completely off the map in 2012, while XP will still account for 15 million units in 2012 and 8.4 million in 2013. Those units will be users who buy Windows 7 and downgrade the OS to XP via rights in their Software Assurance contracts, says Al Gillen, the IDC analyst who tabulated the Windows OS forecasts.

"What happens is Windows 7 comes along and it immediately co-ops Vista's momentum. Vista momentum pretty directly shifts over to Windows 7," says Gillen.

That Windows 7 momentum will translate in 2013 to the new OS accounting for 95 per cent of the operating systems Microsoft sells to businesses. That percentage is up from 90 per cent forecast for 2012.

"At some point users are going to want to move forward," says Gillen. "All you need is demand for one application designed for Windows 7 and you can no longer use XP."

Gillen says users are going to get ‘trapped' between two products [XP and 7], and once that happens it makes more sense to go forward not backward.

Gillen also says Windows 7's XP Mode also will help the transition for users that still have XP applications.

"And for those customers that moved to Vista, it makes sense to me for them to move forward to 7 on a fairly immediate basis," he says.

Microsoft is expected to have the final code ready to ship on October 22.