Microsoft Windows Vista operating system (OS) managed to substantially increase its install base in September, whereas both Windows XP and Windows 2000 saw their market share decreased. While Apple Macintosh platforms also increased their share last month, the growth for Vista was considerably higher.

In September the share of Windows Vista-based personal computers used to browse the Internet was 7.38%, up more than one percent from August, according to data collected by Net Applications, a provider of Web tools. The shares of Windows 2000 and Windows XP operating systems decreased to 3.32% and 79.32%.

If Windows Vista continues to gain momentum in the coming months, in December considerably more than 10% of personal computers will use the new OS from the world’s largest maker of software. However, further Vista’s market share increase may be restrained by Microsoft’s decision to allow customers to downgrade Vista to XP and to order the XP till mid-2008.

The share of systems based on Mac OS increased to 6.61% in September, up considerably from August’s 6.15%, which could be a result of successful sales of new Apple Macintosh computers in August and September as well as seasonality. “Other” operating systems, namely outdated Windows versions, Linux OSes and so on, which have been losing market share for many months now, are now used in 3.39% of devices that are used for Internet browsing.

It was unclear at press time how many computer systems Net Applications tracked in September, 2007.