Along with the Office franchise, it generates the majority of the company's revenues. But one day, the company could "open source" the code that underpins the OS—giving it away for free. So says Mark Russinovich, one of the company's top engineers.

"It's definitely possible," Russinovich says. "It's a new Microsoft."

Russinovich is sitting in front of several hundred people who spend their days running thousands of computers. He helped build Windows, and he carries one of the most respected titles at the world's largest software company: Microsoft Technical Fellow. But here, on stage at a conference in Silicon Valley, he's perched in front of an audience whose relationship with Microsoft is, at best, complicated.