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Previously the service pack could only be installed manually, but from last night PCs began alerting users that the update is ready. Not all Vista users will see it immediately, as Microsoft is phasing it in.

SP1 was completed in February, only for Microsoft to discover problems with a number of third party device drivers. The company tweaked Windows Update to ensure that systems running these drivers do not install the Vista update.

Mike Haigh, Windows client marketing manager, said that the problem with such drivers is to be expected, since Vista has to support so many devices, but "the overall device driver story is a great one" with Vista supporting 80,000 devices.

Microsoft has also released the latest batch of extras for users who bought Vista's Ultimate Edition: two new sound schemes and three resource-hungry animated desktops available from ultimatepc.com.

And for users still on XP, service pack 3 should arrive next week.