Microsoft (NSDQ: MSFT)'s Windows Vista operating system is proving far less popular with new PC buyers than Windows XP did during XP's first year on the market, if statements by company chairman Bill Gates at this week's Consumer Electronics Show are any measure.

Gates, in Las Vegas Sunday, boasted that Microsoft has sold more than 100 million copies of Windows Vista since the OS launched last January.

While the number at first sounds impressive, it in fact indicates that the company's once dominant grip on the OS market is loosening. Based on Gates' statement, Windows Vista was aboard just 39% of the PC's that shipped in 2007.

And Vista, in terms of units shipped, only marginally outperformed first year sales of Windows XP according to Gates' numbers -- despite the fact that the PC market has almost doubled in size since XP launched in the post 9-11 gloom of late 2001.

Speaking five years ago at CES 2003, Gates said that Windows XP in its first full year on the market sold more than 89 million copies, according to a Microsoft record of the event.

Assuming Gates is using consistent measurements across time -- and any failure to do so would raise questions about Microsoft's reporting tactics -- first year Vista unit sales have exceeded first year XP unit sales by little more than 10%.

Windows XP launched in October 2001. According To Gartner Dataquest, worldwide PC shipments in 2002 totaled 132.4 million units. Windows Vista launched in January of 2007 -- a year in which PC shipments will have totaled 255.7 million units when the final tallies are in, according to Gartner.

Gates' statements at the 2003 and 2008 Consumer Electronics Shows thus reveal -- calculating roughly -- that Windows XP captured about 67% of the new PC market during its first year. Vista, by contrast, captured just 39%, or less than half, of new PC shipments in 2007.

The numbers are no doubt troubling for Microsoft, which spent millions of dollars developing and promoting Windows Vista.

Despite the efforts, many corporate and individual PC users have turned their backs on Vista -- citing concerns about its resource requirements and compatibility with older applications.

A survey published by InformationWeek last year revealed that 30% of corporate desktop managers have no plans to upgrade their company's PC's to Vista -- ever.

Contributing to Vista's woes is the fact that new desktop alternatives to the Windows operating system have emerged in recent years -- including Apple's beefed up Leopard OS and open source offerings from Ubuntu and other Linux distributors.