Microsoft has admitted that it sent a Vista patch to the wrong users. The company has also updated the preview release of Vista Service Pack 1 (SP1) to a small group of testers.

The update, which is described in the support document KB935509, was one of three prerequisites for SP1 unveiled Tuesday, and was supposed to end up only on Vista Enterprise and Vista Ultimate machines, since it targeted BitLocker, the full-drive encryption technology bundled with those premium versions of the operating system. Instead, the update was also offered to PCs running Vista Home Basic and Home Premium.

"We had a small number of early customer reports, that in some cases, this update was being offered for installation on all Windows Vista editions versus just Ultimate and Enterprise," said an anonymous poster on the Microsoft company blog devoted to the Windows Update development team.

"For systems set to download and install updates automatically, the update will not install even if it has already downloaded, so most people will not be affected by this," the post continued. "Customers who installed the initial release of the update on editions other than Ultimate or Enterprise should not be concerned as the update will have no negative impact on their systems."

The company has also updated SP1. "Microsoft [has] released the latest prerelease build of SP1, Windows Vista SP1 RC Refresh, to approximately 15,000 beta testers," a Microsoft spokeswoman said. "This group includes corporate customers, consumer enthusiasts, software and hardware vendors, and others. The code is not available for public download."

Four weeks ago, Microsoft made Vista SP1 Release Candidate available to the general public for the first time. The 15,000 testers, however, had earlier beta versions to work with, as well as this most recent update.

This week's glitch was the latest in a series of Windows Updates snafus that include the September revelation that, contrary to users' instructions, Windows' update code had updated itself on their PCs, and charges in October that the company's OneCare security suite was also monkeying with users' update settings. Microsoft denied doing anything untoward with OneCare.