Microsoft has sold four million upgrades for Windows 8 in the last three days, the company announced this morning during its BUILD 2012 conference.

Steve Ballmer, the CEO of Microsoft, greeted crowds at the highly-anticipated event and said that enthusiasm for the new operating system was “very high”. The new figure is understood to include individual purchases, as well as units that have already been sold to stores and which they expect to sell.

In comparison, Apple shipped three million copies of its latest operating system for Mac, known as Mountain Lion, in just four days after its launch.

Those figures may look similar, but of course with a much larger install base of existing Windows users, Microsoft be hoping for a much larger surge in conversions to Windows 8 over the coming weeks. In percentage terms, they’re far behind Apple’s conversions, but their number should increase further as users look to purchase new devices which already have Windows 8 installed.

ballmer crop Microsoft announces 4 million Windows 8 upgrades sold in first three days

Balmer also revealed at the BUILD 2012 event, which is being held at Redmond in Washington, that “tens of millions” of Windows 8 copies had been sold to its OEM partners. Oh, and during the event he also revealed that the SDK for Windows Phone 8 was live.

The Windows 8 launch came and went on October 26, with some pretty comical people promoting the new operating system, and we’ve since been looking at the reaction to Windows 8 both from consumers and app developers to try and get an idea of what the immediate take-up will be.

A slide used by Ballmer, titled “A spectacular start”, had the following quote from Mark Slater, a Category Director at Dixons Retail: “Sales of laptops during launch weekend have been 20% ahead of our forecasts.”

John Solomon, senior vice president of Americas sales for HP’s Printing and Personal Systems division, was also quoted saying: “We are excited about what we are seeing from initial sales and it looks to be good.”

Only time will tell if the level of demand for the Windows 8 operating system will continue.