Microsoft is making a big bet on smartphones, but many customers complain that there isn't the same diversity of apps available on Windows phones as other popular devices like the iPhone.

But the company insists it's catching up. Microsoft CEOSteve Ballmer made a surprise appearance Monday night at the Consumer Electronics Show in Las Vegas to tout the big increase in the number of apps available to Windows phone users.

Ballmer said Microsoft sold five times more Windows Phone devices during the week of Christmas than the same period a year earlier. He didn't give specific numbers.

One challenge for apps developers is the relatively small market share for Windows phones. With fewer potential customers, it's hard for companies to justify the expense of making Windows versions.

One Seattle-area exception is Extended Results in Redmond. The company built a Windows app for its service that crunches corporate datasets for big businesses.

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