Troubled Servicing Stack Update code flipped the reboot switch on unsuspecting users, Microsoft says.
Microsoft has fixed the issue of endless reboots that some customers had
while installing the Servicing Stack Update that contains the installation
program for Windows Vista SP1.
That issue led Microsoft to suspend the automatic distribution of the SSU pending an investigation into the problem.
"Over the past few weeks, we've learned a lot more about the problem and have taken steps to address the
issue. Today, we'd like to let you know that we are resuming automatic
distribution of the SSU tomorrow and provide more clarity on what
happened," said an April 7 post on the Microsoft Update Product Team blog.
The team also stressed that, for those who have already successfully
installed the update, there are no problems with the files that make up the
SSU, as the problem was with the installation process for the update. "If
you already have the update installed, you do not need to uninstall it or
install the rereleased version of the update," the blog post said.
Troubled SSU code
As to the cause of the problem, that lay with the special SSU code that
checked to see if there were any pending reboots or other updates to install.
If it found either of these, the installation could not start, the team said in
its post.
"There were also a few unknown and rare events that occurred 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," the post said.
Microsoft will introduce a fix for this April 8 that will install before the
SP1 Servicing Stack Update does, and which works to prevent the system from
rebooting during the SP1 SSU installation. Again, this will apply only to those
customers who have not already installed the SSU.
"We also made additional changes to the SSU installer code, so that it
checks for and requires the pre-SSU before it will install. These two updates
should now install seamlessly through Windows Update, in the proper
order," the team said.
Customers who get their updates installed
automatically through Windows Update and who have not yet installed the SSU are
not required to take any corrective action, while the stand-alone download of
SP1 has not been affected by the SSU issues, the blog post noted.
/ 4 
