As you know, Steve Ballmer, the "infamous" CEO of Microsoft has announced that he'd be retiring within a year. In an interview with ZDNet, 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. 
Q: Your biggest regret?
Ballmer: Oh, you know, I've actually had a chance to make a lot of mistakes, and probably because, you know, people all want to focus in on period A, period B, but 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. I would say that's probably the thing I regret most. And, you know, there are side effects of that when you tie up a big team to do something that doesn't prove out to be as valuable.