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 percent of all systems that browsed the Web, up from January's 0.18 percent. Vista has grown its usage share each month since its release to consumers Jan. 30, hitting 0.93 percent in February, 2.04 percent in March, 3.02 percent in April and 3.74 percent in May. Apple Inc.'s Mac OS X, meanwhile, accounted for 6.22 percent in January and hit its high point of 6.46 percent in May, but it slipped back to 6 percent 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 percent of the machines Net Applications tracked, while the Intel version accounted for just 1.88 percent. 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 percent and Intel editions at 2.48 percent.

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 percent of all machines that month but was down to 81.94 percent in June. Windows overall total has remained flat, ranging between 90.01 percent and 90.46 percent 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 percent 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.