Microsoft has flipped the switch to start automatically upgrading copies of Windows Vista to Service Pack 1 via Windows Update. Not everyone will be offered the update immediately, however, as the company is distributing it in phases to “ensure a seamless download experience” – or in other words avoid a situation where thousands of users find themselves stuck in an endless reboot cycle.

Vista SP1 was made available for download to customers last month, though, so users can still obtain SP1 by downloading a much larger standalone installer from Microsoft’s site if the service pack is not visible in their Windows Update.