Microsoft has said it will resume automatic distribution of a Windows Vista update today that two months ago sent some users' PCs into an endless wave of reboots.

The company assured users ahead of the release that it has fixed the problem and it is safe to download the update, one of two prerequisites needed before Vista Service Pack 1 (SP1) can be installed. Besides re-releasing the update for automatic download and installation, Microsoft will also issue a fix to prevent reboots from overwhelming the PC.

"These two updates should now install seamlessly through Windows Update, in the proper order, so those of you with [Windows Update] set to 'install updates automatically' who haven't already installed the [Servicing Stack Update] don't have to take any further action," said an unidentified company employee on the Microsoft Update team's blog.

On Feb. 12, Microsoft began pushing the Servicing Stack Update (SSU) and one other prerequisite to Vista users as the final stage of a two-month process of preparing the operating system for the release of SP1. Within days, however, users flooded Microsoft's support newsgroups with tales of stymied updates and locked-up computers. When these users switched off their machines to regain control, the systems rebooted endlessly.

Microsoft yanked the SSU from automatic distribution as a short-term solution, although it left it on the Windows Update servers.

Without SSU installed, users have been unable to download SP1 through Microsoft's update service. The lack of SSU, however, was just one of several reasons why many users grew frustrated over their inability to download and install the long-awaited service pack when it was posted last month to Windows Update.

The company also revealed more information today about the root cause of the reboot snafu. "SSU has special code to check whether there are any pending reboots or other updates to install," said the blog post. "If it sees either of these circumstances, it prevents the install from starting.

"During our investigation, we discovered that there were a few unknown and rare events during the middle of the installation of the update that could cause the update to think it needed a reboot to complete the installation. If this happened, the system entered a repeating reboot loop."

As it has previously, Microsoft today downplayed the extent of the problem, saying that "several million customers installed the updates successfully" while only "a few customers" lost control of their PCs to the reboots.

The pre-SSU update to be released today should prevent the PC from rebooting during the subsequent SSU install, Microsoft said.

Today is also Microsoft's general security update day for the month. According to a notification published last week, the company will unveil eight security bulletins tomorrow to patch Windows, Office and Internet Explorer.