Windows Vista's share of online users has increased every month this year, while rival Mac OS X -- to which Vista has often been compared -- has shown little, if any, growth, a metrics company reports.

According to Net Applications, in June Windows Vista accounted for 4.52 per cent of all systems that browsed the Web, up from January's 0.18 per cent. Vista has grown its usage share each month since its release to consumers Jan. 30, hitting 0.93 per cent in February, 2.04 per cent in March, 3.02 per cent in April and 3.74 per cent in May. Apple's Mac OS X, meanwhile, accounted for 6.22 per cent in January and hit its high point of 6.46 per cent in May, but it slipped back to 6 per cent in June.

If Vista's uptake trend continues, it should pass Mac OS X in Web usage share by the end of August.

Shifts from the Mac OS X running on older PowerPC processors to the edition running on Intel CPUs have been constant, but according to Net Applications' data, the computer maker is not making new Mac converts. In January, for example, the PowerPC version of Mac OS X was reported by 4.34 per cent of the machines Net Applications tracked, while the Intel version accounted for just 1.88 per cent. June's numbers showed that the PowerPC edition of the operating system still held the lead, but the gap had narrowed considerably: Older versions were pegged at 3.52 per cent and Intel editions at 2.48 per cent.

Likewise, Vista's increases have come at the expense of Windows XP and Windows 2000, both of which have dropped in usage since January. Windows XP, for instance, accounted for 85.02 per cent of all machines that month but was down to 81.94 per cent in June. Windows overall total has remained flat, ranging between 90.01 per cent and 90.46 per cent through the first six months of the year.

Net Applications' 6 percent share for Mac OS X jibes with recent IDC sales estimates, which put Apple's portion at 5.6 per cent of all personal computer sales in the U.S. during the second quarter.

Apple's next major upgrade to Mac OS X, dubbed Leopard, is scheduled to ship in October.