ZDNet: According to Net Applications, Microsoft's Windows XP remained the most used operating system in April, accounting for 46.08 percent of the market. Windows 7 came in second with 38.67 percent market share. Windows Vista, OS X and "other" each had less than 10 percent of computer users.
If Windows XP use continues falling and Windows 7 use continues rising at the same rates, it may take until 2013 for Windows 7 to capture first place in Net Applications' reports.
StatCounter uses a different method to calculate OS market share, and it says that Windows 7 has already overtaken Windows XP.