Microsoft may have a tough time building significant market share for its new Internet Explorer 9 (IE9) browser because eligible users are in the minority.

Several analysts agreed that Microsoft has its work cut out for it, at least in the short term, because IE9 won't run on Windows XP, the aged-but-still-dominant operating system.

Microsoft omitted the still-popular XP from the supported OS list because, among other things, IE 9 speeds up page rendering and composition by tapping the graphics processor in newer PCs. Windows XP lacks support for the Direct2D API, which IE9 uses to accelerate content rendering.

The decision means that only a subset of machines will be able to run IE9, either in its preview or final form, until those systems and Windows XP are replaced by new hardware and Windows 7.

"That's an issue in terms of growth, at least for the next 12 to 18 months as more companies and consumers migrate to Windows 7," said Al Hilwa, an analyst with IDC. For that reason and others, Hilwa said not to expect a "tidal wave change in browser share. Microsoft's battle is not to lose much more share."

Worldwide, Windows XP PCs outnumber all other Windows-equipped machines by two to one, according to the most recent statistics from Web metrics company Net Applications.

In August, Windows XP powered 66.7% of the globe's Windows systems that went online in the month, while the three-year-old Vista accounted for 15.3% and 2009's Windows 7 for 17.4%.

Older versions of Windows, including Windows 2000, Windows 98 and Windows Millennium, made up the small remainder.

Even in the U.S., where Windows XP has a smaller slice of the Windows pie, the aging operating system remains in the majority. But its margin is much narrower. Here, Windows XP ran on 52.1% of all Windows PCs used to browse the Web last month, while Vista accounted for 27% and Windows 7 for 20.8%.

Windows' total share in the U.S. is significantly lower than its global share -- 83.5% for the U.S., 91.3% worldwide -- in part because Apple's Mac OS X is more popular in the United States than in most other countries. Net Applications, for example, pegged Mac OS X with a global share of 5% in August, but a U.S. share of 11.2%.