In an attempt to help people grasp and understand the changes, Microsoft is expected to spend an estimated $1 billion promoting Windows 8.
If Windows 8 is a hit, it could also help lift the fortunes of struggling PC makers, including Hewlett-Packard Co. and Dell Inc., whose stocks have plummeted in recent years amid the rise of mobile computing.
If Windows 8 is a flop, however, it will increase the pressure on Ballmer. Although Microsoft is far larger than when Ballmer became CEO nearly 13 years ago, the company's stock has lost nearly half its value as Apple, Google and Amazon steered computing in a new direction. Restless shareholders could start clamoring for Ballmer's ouster if Windows 8 doesn't shake up the state of the technology market as dramatically as Ballmer envisions.
Microsoft shares fell two cents Thursday to close at $27.88.