Microsoft's (MSFT) Windows 8 operating system is being adopted much slower than the company's last new operating system three years ago. Three months after its official launch, Windows 8 has 2.3% market share, Net Applications reported Friday. It is the fifth most popular personal computer operating system in use. In December, Windows 8 had 1.8% market share. It made its retail debut on Oct. 26, but had been available to corporate users since