Windows 8 grabbed 5.1% market share in June, up from 4.3% in May, finally pipping Vista to the spot as the third most-used Windows OS.
Vista's share slid to 4.6%, while Windows 7 holds 44.4% and XP still has 37.2%. The most recent version of Apple's OS X (10.8) has 3.1%, the analytics firm said.
Vista actually climbed slightly on-month, but has been on downward trend over the past year, sliding by two percentage points since June 2012. XP is down five points since then, while Windows 7 is up three points over the year.
Windows 8's uptake is slower than Windows 7 was at launch, according to analytics firm StatCounter, which shows Windows 7 hitting the 5% mark only a month after release, and topping 18.8% over the same period that Windows 8 has managed a 5.1% slice of the market.
Windows 8's struggles to convince users to upgrade has led Dell to worry about the effect it's having on PC sales.