Microsoft delayed the general release of the next version of Windows until January, potentially wiping out holiday sales for computer makers and crimping revenue at the world’s largest software maker.

The new Vista version of Windows, which runs 90% of the world’s personal computers, is already two years late and Chief Executive Officer Steve Ballmer said as recently as last month that the release was on target for this year.

“This is a major slip with material consequences,” the president of San Jose, Calif.-based researcher the Enderle Group, Rob Enderle, said. “Missing the fourth quarter will have a significant impact on consumer sales.”

Personal-computer makers, who have waited five years for the next Windows, had been relying on Vista to fuel demand in the holiday shopping season, which accounts for 30% of purchases. The delay is also a blow for Microsoft, which gets 31% of its revenue from Windows for PCs.

Sales growth at the company has fallen to its lowest ever while customers wait for the new version. Sales in the Windows unit rose 6% in the year ended June 30, down from 11% growth in the prior year.

Microsoft shares fell 69 cents, or 2.5 percent, to $27.05 in extended trading after the announcement. The stock, which had fallen in the past two years, had gained 6.1% this year, in part because of optimism for the new Windows.

Microsoft held up the release of Vista so it could have time to improve security and address other quality concerns, the co-president of the division in charge of Windows, James Allchin, said on a conference call.

Windows Vista will be introduced in two stages: a shipment to businesses in November this year, and a release to consumers in 2007. Microsoft is trying to “crank up” the security level of Vista, which needs a few extra weeks, Mr. Allchin said on the call.

“We’re trying to do the responsible thing here,” Mr.Allchin said.”Maybe in the past we would have just gone ahead but now we’re not going to do that.”

Some PC makers asked Microsoft to delay the product until it was free of security and quality glitches, Mr. Allchin said. Many may have felt they had little choice, an analyst at Endpoint Technologies Associates, Roger Kay, said.

“For PC manufacturers, it’s like frying pan or fire,” said Mr. Kay, who is based in Wayland, Mass. “Do you want something that’s not secure or something that’s delayed? If you have to choose between those evils, you’d choose delayed.”

At a conference in New York last week, Mr. Ballmer dodged questions about the specific timing of a release of Vista. In a February interview, Mr. Ballmer signaled that the new Windows would be in stores in November, in time for the holiday shopping season.

“We are heads down to ship this year and all of our partners, the retailers, our hardware vendors, will tell you if we’re not in stores before Thanksgiving in the U.S. anyway that’s not a good thing,” Mr. Ballmer said in the interview in Barcelona.

Some analysts had expected Vista even earlier than Thanksgiving, which falls on November 23. Goldman Sachs’s Rick Sherlund, the top-ranked software analyst by Institutional Investor, said in January that the product may be on shelves by mid-October. A January launch will not only hurt fourth-quarter PC sales but may make Microsoft vulnerable to more delays because of holiday vacations, Mr. Enderle said.