Microsoft said Tuesday that the consumer version of Windows Vista, its biggest product release in years, will be delayed until January, missing the important holiday selling season.

A version of Vista for business customers will arrive in November, Microsoft co-president Jim Allchin said during a conference call with reporters and analysts. The company had originally planned to broadly launch Windows Vista, the first update to its $12 billion-a-year desktop operating system business since 2001, by the end of this year. Microsoft decided to stagger the release after PC makers said they couldn't be sure Vista would run on all their new computers in time for the Christmas rush, Allchin said.

"The industry was asking for more certainty," said Allchin. "We made a decision to move the date so everybody could be aligned."

Companies that buy Windows through Microsoft's corporate licensing programs have a choice of two editions of Vista designed for large businesses in November. But three additional editions of Vista--including a package called "Windows Vista Ultimate" aimed at small businesses--won't be available until January. The changes could confuse buyers by making it unclear for the rest of the year which PCs will be able to run the new operating system, which introduces new features for data security, filing, and searching for information, says Michael Cherry, an analyst at research and consulting company Directions On Microsoft.

"This makes it really difficult right now for people who need to buy a new computer," says Cherry. "You don't know what to purchase that's capable of running Vista." Microsoft's earlier decision to sell Vista as six products with unique sets of features risks confusing the market more. "Microsoft hates to be compared to Apple [Computer], but there's only one OS X," he says.

During the conference call, Allchin said staggering Vista's release schedule won't affect the company's forecast for PC sales. Microsoft has forecast 12% to 14% growth in PC unit sales for its fiscal year which ends in June. The company will also go ahead with a planned second beta release of Vista by the end of June. Allchin said PC makers had asked for more time to make sure Vista's security features and other capabilities run smoothly on their products.