For the year, IDC said 269 million PCs were shipped in 2007, up 14.3 percent from 2006. Gartner: 271.2 million units shipped, also a 13.4 percent increase.

IDC analyst Loren Loverde expects weakness in the broader economy to take its toll on the PC market. He said in a statement: "Despite fourth quarter strength, projections for the next couple years anticipate slower growth. Rising concerns about economic growth are likely to reduce expectations further, although we're still likely to see double digit growth through 2008 and probably 2009."

Back to the fourth quarter, growth in the U.S. of 8.8 percent exceeded IDC's expectations. The research firm pointed to expansion of retail sales by Dell and Acer and competition, which is lowering prices. Strong U.S. sales could have a disproportionate impact on Microsoft's bottom line because the company already has a huge base here and piracy rates are lower than in some faster-growing regions.

In October, Microsoft predicted revenue for the second quarter would be $15.6 billion to $16.1 billion, generating operating income of $5.9 billion to $6.1 billion, or 44 to 46 cents per share.

IDC's estimates quantify Apple's resurgence in the year since Microsoft launched Windows Vista. The company shipped an estimated 4.08 million computers in the U.S. in 2007, good enough for 5.8 percent of the market. Only Dell and Hewlett-Packard had a bigger slice of the pie. And Apple's growth rate of 31.3 percent over 2006 was the fastest of any major company except Acer, which integrated with Gateway during the period.