in brief Microsoft is resuming automatic distribution of a SP1 prerequisite update, which sent some Vista users into an endless reboot cycle during installation.
The problematic update — Servicing Stack Update (KB937287) — was pulled from the automatic update in February.
However, now Microsoft has the problem in hand according to the Microsoft Update team blog: "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."
A fix is being released today which will install prior to the problematic update, and prevents the system from rebooting during installation. Changes have also been made to the Servicing Stack Update installer code so it checks for the fix before installing. Both updates will install in the proper order through Windows Update.