Developer Year in Review: Operating Systems: Vista never managed to crack the 20 percent adoption mark, even after four years on the market (it peaked just shy of 19 percent). By contrast, Windows 7 is already past the 20 percent mark, after only a little more than a year. XP, however, is still holding on to more than 50 percent of the Windows market. Not bad for a nine-year-old OS that isn't supported by Microsoft anymore.