For the first time in ages, the sale of new PCs with Windows as a percentage of the PC market is declining sharply. The new winner is the Mac, but, while no one does a good job of tracking the still-new, pre-installed Linux desktop market, it’s also clear that Linux is finally making impressive inroads into Windows’ once unchallenged market share.

The Mac numbers are especially revealing. NPD, a global market research company, has revealed that Apple’s share of the U.S. computer market jumped to 14 percent in February 2008. This was up from 9 percent in February 2007.

In comparison to the overall market, U.S. PC retail shipments only grew 9 percent in units shipped and a mere 5 percent in revenue in the last year. Macs, in the meantime, saw a 60 percent growth in unit sales with an even more impressive 67 percent gain in revenue growth over the same period.

In an investors’ research note based on the NPD data, Pacific Crest Securities analyst Andy Hargreaves said that Apple did especially well with its laptops. Better still, though, “MacBook Air sales appear to be additive to total sales, rather than replacing MacBook Pro sales. We believe a new set of corporate customers makes up a meaningful portion of MacBook Air buyers,” said Hargreaves.

At the same time, while, to the best of my knowledge, no analysis firm is tracking Linux desktop sales yet, empirical evidence makes it clear that Linux desktops are moving into customers’ hands at a quick pace.

For example, according to the Amazon’s Bestsellers: The most popular items in Computers and PC Hardware list of March 21, the best-selling computer is not a Mac. It’s the Asus Eee PC 4G Surf. Simultaneously, other retailers, such as Best Buy and Sears, are also offering low-priced Linux PCs.

While HP still hesitates from being the last of the major PC vendors to commit to desktop Linux, others are predicting great things from the Linux desktop. Asus, the Taiwanese OEM (original equipment manufacturer) behind the Eee Linux PC line, is predicting that it will sell 2 million Linux PCs in 2008.

I see two strong trends here. On the high end, people are buying Macs instead of Windows PC. On the low end, Linux is eating Windows alive.

Windows finds itself being confined to the middle ground.

This is happening because Vista, SP1 and all, has proven to be a commercial flop. Even Microsoft seems to be backing off Vista and looking ahead to its next operating system, Windows 7.

I’ve long predicted that Vista would be a failure and that this failure would give the Linux desktop its shot at the big time. That’s no longer a prediction.

It’s an observation of the facts.

This year is proving to be the year of desktop transformation. I see Macs continuing to gain strength with users who want the best of the best and are willing to pay the price. At the same time, the low end is going to be completely owned by Linux. From there, thanks to PCs and laptops from Dell, Lenovo and, eventually, HP, Linux will start moving up the PC ecosystem. Microsoft will be forced to a very unusual position: a defensive battle.

How Microsoft deals, or fails to deal, with this new challenge will determine if Windows continues to be the dominant desktop operating system. Distracted by its Yahoo buyout plans, its courtroom failures and its leadership changes, Microsoft is in trouble, and that means the other desktop operating systems have their best chance ever to knock Windows off the top of the desktop hill.