Microsoft's Windows OS could play a crucial rule in returning worldwide PC shipments to modest growth next year after multiple years of decline, Gartner said on Monday. PC shipments could reach around 317 million in 2015, increasing from 308 million units expected to ship this year, the research firm said in a study. Shipments this year are expected to decline by 2.9 percent compared to 2013, which is lower than previous yearly declines.