A. The easiest way I've found is to use statcounter.com, which has a page on OS deployment. The pages shows that from July 2009 to August 2010, Windows 7 has overtaken Windows Vista . We still have a way to go before it overtakes Windows XP, however. In December 2009, Windows 7 overtook Mac OSX, even though it had only been out one month .
Note that this site is based on sites that use the StatCounter services, but it's still pretty representative of the overall market.
Other statistics are available, such as mobile OSs, web browsers, and search engines. It's pretty interesting.