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You can download Windows Vista SP1 from Windows Update. SP1 will start downloading to PCs automatically beginning in mid-April. The launch of SP1 was delayed to help insure support for drivers.
If you are running Windows Update, but don't see Windows Vista SP1 there are a number of reasons for this. In fact, there are eight reasons. See the whole list at the knowledge base article: Windows Vista Service Pack 1 is not available for installation from Windows Update and is not offered by Automatic Updates Typically, you may not have all of the prerequisite packages. To install them, go to the Windows Update control panel and click Check for updates.
For more details about Windows Vista SP1, see the Windows Vista team blog entry at Windows Vista SP1 released to Windows Update. Also available