Steven Sinofsky, who led Microsoft's Windows division before abruptly leaving the company in late 2012, is dipping a toe into the world of startup investing. This morning, Sinofsky announced on his blog that he was joining Andreessen Horowitz, one of the biggest venture capital firms in Silicon Valley. "I'm relatively new to the VC world and have a lot of learning to do — and I am very excited to do that," he writes. Sinofsky served as Windows team president starting in 2009, after two decades at Microsoft, but since his departure he's launched a productivity and development blog called "Learn by Shipping" and begun a teaching stint at Harvard Business School.

Sinofsky writes that he became acquainted with firm founders Marc Andreessen and Ben Horowitz late last year, and he's become interested in moving into venture capital. He's not, however, going to be joining the firm full-time. He'll serve instead as a "board partner," a position that will let him periodically join the boards of companies that receive funding from Andreessen Horowitz. While Sinofsky was frequently described as a laser-focused, sometimes abrasive Windows head, we're told that his new role will be open-ended, as he moves between various startups and the core Andreessen Horowitz team.

Some of the projects Sinofsky picks will play to his interest in productivity software, but in other cases, he'll simply be bringing his management experience to an unrelated project. It's not clear yet how much time he'll actually be spending at the company, but we've heard that he's already considering a project. At least for now, though, it's clear that he's not considering switching over to full-time work. In addition to the blog and his position at Harvard, Sinofsky says that he "will continue to pursue other business and product development opportunities that arise."