Microsoft's strategy of targeting first time smartphone buyers appears to be paying off, with Windows Phone now standing as the fastest growing OS on the market.That's according to Kantar Worldpanel, which claims Windows Phone's momentum is building after a slow start, with its market share in major markets such as the US, UK, France and Italy in particular growing at pace."As iOS and Android continue to battle it out for top selling smartphone OS, we have seen Windows steadily grow over the past year and is now at its highest sales share figure so far," said analyst Mary-Ann Parlato of Kantar Worldpanel's results."Windows strength appears to be the ability to attract first time smartphone buyers, upgrading from a feature phone. Of those who changed their phone over the last year to a Windows smartphone, 52 percent had previously owned a feature phone."Comparatively, the majority of iOS and Android new customers were repeat smartphone buyers, with 55 percent of new iOS customers, and 51 percent of new Android customers coming from another smartphone."The end result, Kantar Worldpanel claims, is 5.6 percent market share in the US, up 1.9 percentage points from the same period in 2012.While some distance between Android and iOS  on 49.3 percent and 43.7 percent respectively  it's Microsoft's position against its 'third place rival' BlackBerry that's most relevant.In the US, for instance, BlackBerry's share has fallen from 2.9 percent in March 2012 to less than 1 percent now  its share falling at roughly the same pace as Windows Phone's is growing.It's a pattern that's mirrored in the UK, where BlackBerry has dropped from a more than respectable 15.3 percent share of the smartphone market to a much more modest 5.1 percent.In comparison, Microsoft's has engaged in a marketing blitz in the region, growing its base to a 7 percent share  that's more than twice the slice of the pie Windows Phone accounted for in 2012.One caveat to the figures, however, is that  in the US at least  they don't take the impact of the BlackBerry Z10 into account, which only made its debut in mid-march.Nonetheless, Kantar Worldpanel believes Microsoft's continued efforts to target first-time buyers with celebrity-based ad spots is likely to push its share even higher in the months ahead."While the differences between these figures are small, with over half of the US market still owning a feature phone, its likely that many will upgrade over the coming year, which will ultimately contribute to more growth for the Windows brand," concluded Parlato.Microsoft will be hoping Windows Phone can mirror Android's ascent, which  while now dominant, accounting for almost half of smartphone consumers in most major territories  sported single figure market share as BlackBerry and Apple tussled it out at the top until 2010.[source: Kantar Worldpanel