Steve Ballmer is retiring from Microsoft within the next 12 months, and he's had some time to reflect on his 13-year experience as CEO. In an interview with ZDNet's Mary Jo Foley, Ballmer reveals that his proudest achievement over the 33 years he spent at Microsoft was being "a significant part even of the birth of intelligent personal computing." Detailing his regrets, he lays the blame solely on Windows Vista. "I would say probably the thing I regret most is the, what shall I call it, the loopedy-loo that we did that was sort of Longhorn to Vista," says Ballmer. "I would say that's probably the thing I regret most."

That "loopedy-loo" saw Microsoft reset the codebase of Windows Vista midway through its development, dropping a number of ambitious features that it promised during a developer event in 2003. Ballmer has previously discussed the long development process of Windows Vista, noting that "we tried too big a task and in the process wound up losing thousands of man hours of innovation." Windows Vista, codenamed Longhorn, was originally expected to debut just two or three years after Windows XP. A complicated development process meant it shipped five years after XP, and missed the all-important holiday season for retail availability.

What will Ballmer do next?

While Ballmer regrets Windows Vista, his plans for his post-Microsoft future are unclear. "Frankly I don't know," he says, noting he hasn't had much time to consider his plans. "My whole life has been about my family and about Microsoft. And I do relish the idea that I'll have another chapter, a chapter two, if you will, of my life where I'll get to sort of experience other sides of life, learn more about myself, all of that, but it's not like I leave with a specific plan in mind."