Windows XP slid 1.3 percentage points in December, its second-largest one-month decline ever. (The record remains November, when XP lost 1.4 points.) Vista, meanwhile, lost 0.7 of a percentage point, a single-month record, to end at 17.9%. December was the second month in a row that Vista lost share, and the third in the last four months, a trend that points to a as users abandon it for Windows 7.

Still, the bulk of Microsoft's losses since the Windows 7 launch on Oct. 22 have been from Windows XP; the eight-year-old OS has lost 2.7 points in the last two months, while Vista has lost only 1 point.

Microsoft's newest OS, on the other hand, boosted its share by 1.7 percentage points to end December with 5.7%, meaning that approximately 1 out of every 18 machines on the Web ran Windows 7 last month. If it can keep up the pace of the last 60 days, Windows 7 will crack 7% this month, beating Vista to that number by six months.

Windows 7 also reached a milestone on Jan. 1, 2010, when it posted an 8% share for the day. The previous one-day record of 7.6% had been set on Dec. 27, 2009.

Apple's Mac OS X dipped for the second month in a row, finishing December with 5.1% after a decline of a statistically insignificant 0.01 of a percentage point. Most months, however, Mac OS X posts gains, not losses: December was only the fifth month of 2009 in which Apple's operating system lost share.