Microsoft CEO Steve Ballmer can't afford to be wrong about Windows 8. On Thursday in New York, Microsoft unveiled a dramatic overhaul of its ubiquitous Windows operating system. It will go on sale Friday, fused into more than 1,000 PCs and other devices. If it flops, the failure will reinforce perceptions that Microsoft is falling behind competitors such as Apple, Google and Amazon as its stranglehold on personal computers becomes less relevant in an era of smartphones, tablets and other mobile devices.