"We've reimagined Windows and we've reimagined the whole PC industry," Microsoft Chief Executive Steve Ballmer told Reuters Television early Thursday ahead of the launch.

Windows 8 devices and the company's new Surface tablet, which aims to challenge Apple's popular iPad head on, go on sale at midnight.

Early reviews of the Surface tablet have been mixed, with praise for its slick hardware, but concerns about battery life and limited software and applications available.

The new design of Windows, which dispenses with the Start button and features square tiles for apps, may shock some users. Initial demand appears solid, but customers are wary.

"We've seen steady pre-order sales on Windows 8 devices from early adopters," said Merle McIntosh, senior vice president of product management at online electronics retailer Newegg. "However, we expect that most average consumers are waiting until after launch to make a purchase decision."

Investors are uncertain about the prospects for success of Windows 8, but many feel a solid launch could help Microsoft's stock, which has languished between $20 and $30 for much of the last decade.

Apple's shares have significantly outperformed Microsoft's over the past 10 years, and its market value is now more than double Microsoft's. Microsoft shares were unchanged in early trading on Nasdaq at $27.92; Apple was steady at $615.55.

"This really is about debunking the notion that Microsoft is a dinosaur and they are relevant in a new climate of tablets and mobile," said Todd Lowenstein, portfolio manager at HighMark Capital Management, which holds Microsoft shares.

"Extreme pessimism and almost utter failure is priced into the shares, so any kind of positive delivery on units, customer perception, would be really beneficial to the stock." (Writing by Bill Rigby in Seattle; Editing by Bernadette Baum)

• A Permanent Economic Emergency