The Windows Vista SP1 update designed to prepare PCs for the imminent release of the service pack has been pulled by Microsoft, following reports that it endlessly rebooted Vista PCs.

Although the update - actually a pair of prerequisite files that modify Vista's install components - has been temporarily pulled from Windows Update, Microsoft has not yet produced a fix for users whose machines either won't boot or reboot constantly.

"Immediately after receiving reports of this error, we made the decision to temporarily suspend automatic distribution of the update to avoid further customer impact while we investigate possible causes," said Nick White, a Vista programme manager, in a post to the company's blog.

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White downplayed the problem. "So far, we've been able to determine that this problem only affects a small number of customers in unique circumstances. We are working to identify possible solutions and will make the update available again shortly after we address the issue."

According to White, Update 937287 was the cause of the problem. In a support document, Microsoft describes that update as one for Vista's installation software, "the component that handles the installation and the removal of software updates, language packs, optional Windows features and service packs".

Along with a companion update pushed to users starting February 12 and another that was offered to machines running Vista Ultimate and Vista Business in January, the guilty update is required before Vista can be upgraded to Service Pack 1 (SP1).

Shortly after the two prerequisites hit Windows Update last week, users began reporting problems on Microsoft's support newsgroups. Most said that the update hung as the message "Configuring Updates Step 3 of 3 - 0% Complete" appeared on the screen. When users rebooted hoping to clear the error, their PCs went into an endless cycle of reboots. A smaller number of users said that their computers refused to boot normally.

Some users have been able to regain control by booting from a Vista install DVD and selecting the "Restore from a previous restore point" option.

It's uncertain whether Microsoft knows exactly why Update 937287 is hammering Vista PCs. Even after White posted the company statement to the Vista blog, Darrell Gorter, a Microsoft employee, was asking users to send him system logs. "I still need more log files for the investigations that we are doing," Gorter said in a message on the support newsgroup. Late last week, Gorter made a similar request on the same message board.

Also unclear is the actual extent of the problem. Although White called the number 'small', the traffic on the Vista SP1 newsgroup is heavy. One thread had been viewed more than 35,500 times by late Tuesday.

But the problem is not new. PC Advisor's sister title Computerworld found messages describing the endless reboot problem dated December 13, one day after it first offered a Vista SP1 release candidate to the general public. That build of SP1 also required the prerequisite updates, including 937287.

Currently, only beta testers, Volume Licensing customers, and subscribers to TechNet Plus and Microsoft Developer Network have been able to download legal versions of the service pack.

That will change in mid-March when SP1 is set to land on Windows Update as an optional update, and again in mid-April when Microsoft said it would start installing SP1 automatically on most PCs running Vista.

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