On Thursday, some Windows Vista users began finding Service Pack 1 in Windows Update, even though the upgrade isn't supposed to be available broadly until the middle of March.

Microsoft acknowledged the error. "Yesterday, a build of SP1 was posted to Windows Update and it was inadvertently made available to a broad group. The build was intended only for our more technically advanced testers, and was meant to only be offered to those with a specific registry key set on their PC," Microsoft said in a statement. It also reiterated plans to make SP1 broadly available in mid-March.

Some customers on a Windows Vista forum reported that they successfully downloaded SP1 from Windows Update, but most others said that the download didn't work for them.

The accidental posting to Windows Update follows another recent issue with an update designed as a prerequisite for downloading SP1. Some users, after trying to install the update, got stuck in a reboot cycle. Earlier this week, Microsoft posted a fix for that problem.

Microsoft issued a second refresh of SP1 to beta users in late January, raising hopes that the final version would be out within a couple of weeks. The company had long said that SP1 would come out in the first quarter.

The final broad release of SP1 could boost Vista sales, particularly among enterprise users, because some companies have said that they are waiting for SP1 before upgrading to Vista.