The Microsoft Update blog provides an update (ahem) explaining how the company is resuming automatic distribution of a prerequisite for Windows Vista Service Pack 1 and releasing a fix which will install prior to the SP1 Servicing Stack Update (SSU). According to Microsoft, this pre-SSU update helps to ensure a smooth install of the SSU by working to prevent the system from rebooting during the SP1 SSU installation, an issue that a small number of customers experienced when they first issued the SSU in February.

Back in February, we posted about the release of a couple of prerequisites for Windows Vista Service Pack 1.  While several million customers installed the updates successfully, you may have read that a few customers experienced an endless reboot cycle while installing one of the prerequisites: KB937287, the Servicing Stack Update (SSU), which contains the Service Pack 1 installation program.

As posted last month on the Windows Vista blog, we suspended automatic distribution of the SSU while we investigated 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.

To clear up any concerns for those of you who have already installed the update: There is no problem with the files that make up the Servicing Stack Update (KB937287); the problem some customers encountered was with the installation process for the update.  That means if you already have the update installed, you do not need to uninstall it or install the rereleased version of the update.  

So what caused the problem?  Well, the SSU has special code to check whether there are any pending reboots or other updates to install.  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.

To address this problem for people who have not already installed the SSU, we are releasing a fix tomorrow which will install prior to the SP1 Servicing Stack Update.  This pre-SSU update helps to ensure a smooth install of the SSU by working to prevent the system from rebooting during the SP1 SSU installation.  We also made additional changes to the SSU installer code, so that it checks for and requires the pre-SSU (KB949939) before it will install.

These two updates should now install seamlessly through Windows Update, in the proper order, so those of you with WU set to “install updates automatically” who haven’t already installed the SSU don’t have to take any further action. For those using the standalone download of SP1, the issues we encountered do not affect that method of installing at all.