Microsoft Corp. (Nasdaq: MSFT) is selling four times as many Windows phones as this time last year, helped by a new version of its phone software for more powerful handsets with faster screens, Chief Executive Officer Steve Ballmer says.

“Windows Phone 8 has been on sale for a few weeks and is off to a great start,” Ballmer said at a shareholders’ meeting Wednesday. Handsets made by HTC Corp. and Nokia Oyj “are getting rave reviews and have initially sold out in many countries.”

Microsoft is overhauling its product line to remain relevant in the post-PC era. The Redmond, Washington-based company dominated the technology landscape during the heyday of the personal computer in the 1990s and much of the last decade, yet has been eclipsed in influence by a resurgent Apple Inc. and the rise of mobile devices taking the place of PCs that run Microsoft’s flagship Windows operating system.

Windows 8 represents Microsoft’s biggest change to its flagship product in nearly two decades — designed around a revamped home screen featuring colorful tiles that launch programs and update users with current information from the Internet. On Oct. 26, the same day Windows 8 went on sale, Microsoft started selling Surface, a tablet computer designed and built by the company.

Ballmer said Microsoft is seeing “fantastic demand” for tablets and touch-screen PCs featuring Windows 8. There are now 1,500 different machines running the software, up from 1,000 when the operating system debuted, he said.

There are now 120,000 applications available for the Windows phone, Ballmer said. He didn’t specify how many phones running the new software have been sold.