Since its release in January, Microsoft's Windows Vista operating system has steadily gained market share at the expense of Windows XP and 2000, while the Mac OS has shown no significant market gain, according to a Web metrics firm.

As of June, 4.52% of the computers Net Applications tracks on the Web were running Windows Vista, while 6% were running the Mac OS, either on older PowerPC processors or Intel's x86 chips. The Mac numbers were in line with a recent global PC market report from International Data Corp., which estimated that the Apple Mac had a 5.6% market share in the United States and had tied Gateway for the No. 3 spot.

Net Applications shows Vista gaining market share steadily, rising from 0.18% in January. In February, March, April, and May, the numbers were 0.93%, 2.04%, 3.02%, and 3.74%, respectively.

For the Mac OS, the numbers were relatively flat. In January, the operating system had a 6.22% share, followed by 6.38% in February, 6.08% in March, 6.21% in April, and 6.46% in May.

Vista's market gains have come at the expense of the older Windows XP and 2000, which have shown a steady month-to-month decline. In January, XP and 2000 had shares of 85.02% and 4.93%, respectively. By June, XP had an 81.94% share, and 2000 had a share of 4%.

Most of Vista's gains have been from consumers buying new PCs, which come with Vista preinstalled. Businesses, on the other hand, have been slow to embrace the new OS, which requires machines with more horsepower than previous versions of Windows. In addition, a significant number of business applications still need to be upgraded to support Vista, and hardware manufacturers are still writing new software drivers for the OS.

Vista has done little to drive PC demand or replacement sales since the OS' release, analysts say. Nevertheless, PC shipments globally are rising and are expected to total 257.1 million units this year, an 11.1% increase over 2006, according to Gartner.