The below graphic answers some of these questions. This dataset was put together by w3schools, and was collected from W3School's log files over a period of seven years. While w3school users do not accurately represent the entire population of computer users, it is the best publicly available dataset that we could find.
Here we see that while Windows 7 has been gaining market share, it has (for the most part) been eating shares away from Windows Vista and Windows XP. We also see that it fared much better than Windows Vista, which never exceeded more than 20% market share. While Windows 7 has more than 50% market share now, usage of non-Windows Operating Systems has been growing since late 2008.
Below is a line chart constructed from the same dataset. While the two charts show the same data, it tells different aspects of the story.
Here we see that Windows XP seemed to be displacing Windows 2000 from the market. Likewise, Windows 7 is displacing the once-popular Windows XP, something that Windows Vista had failed to do. In this representation it is very easy to see where the lines cross: Windows 7 market share overtook Vista just six months after its launch, and XP less than a year after its launch. It'll be interesting to see how Windows 8 fare in the future.
We can use a less verbose way of defining a chart. We demonstrate this with the code for the second chart: