Microsoft CEO Steve Ballmer made a surprise appearance at the end of Wednesday’s BUILD keynote speech, telling the audience that Microsoft is in the process of reimagining itself.

Addressing Microsoft’s future, Ballmer said: “In a sense you could say that if Windows 8 is Windows reimagined, we're also in the process--and Windows 8's an important step of that--of reimagining Microsoft.”

He ticked off four big themes he said the software giant is focusing on: New hardware form factors, cloud services, new application scenarios, and new forms of developer opportunity.

“While it's clear we have a long way to go still with Windows 8, we've been gratified certainly by the reactions and the interest,” Ballmer said.

“I sure hope the show has been interesting. I know it's been fun," Ballmer told the gathered developers. "I went and checked how quickly you activated the machines that you got yesterday, and I guess I could tell you what you were all doing last night.” Each conference attendee received a Samsung Windows Developer Preview PC tablet with the preview version of Windows 8 loaded on it.

Ballmer told the crowd that there have already been 500,000 downloads of the developers’ preview version of Windows 8, which is available to anyone who is interested.