Windows 8 surged in December to end the year with almost 12% of the user share of all Windows personal computers, while the destined-for-retirement Windows XP restarted its decline after a two-month pause, a Web analytics company said Thursday.

Both were good signs for Microsoft, which has bet its future on Windows 8 and implored customers to abandon the aged Windows XP.

According to Net Applications, Windows XP fell 2.2 percentage points in December to 29% of all desktop and notebook computers worldwide, the first time it breached that 30-percent barrier. But the 12-year-old operating system still accounted for nearly a third -- 32% -- of Windows-powered PCs.

Meanwhile, Windows 8's and 8.1's combined user share of all computers reached 10.5%. Of the systems running Microsoft's OS, Windows 8/8.1 owned a user share of 11.6%.

Both operating systems had taken a break in October and November from earlier trends: Windows XP's gradual decline and Windows 8's deliberate growth.

Their December changes were the largest since September, Net Applications data showed.

The gain by Windows 8 and 8.1 was likely due to new PC purchases in the last month of 2013: Most consumer systems come equipped with the newest version, Windows 8.1, which accounted for 34% of the combined total, up from November's 28%.