Mary-Jo Foley got 15 minutes with Ballmer (as well as a number of other reporters), he talked about his biggest regret:
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.
Source: Mary-Jo Foley