Unable to install Vista SP1 from Windows Updates
  On March 18th, 2008 Microsoft release Windows Vista's Official Service Pack 1 (SP1) onto their Windows Update Service and to download through Automatic Updates. Many users have reported that they are unable to download or even see this as an option in their Vista Updates Window. Microsoft has released a list of the most common reasons that users are unable to access this download and some potential solutions that can help fix the problem. The initial list is below, followed by the official Microsoft Knowledgebase link for the issue.
  • You have not yet installed all the prerequisite packages you need for Windows Vista SP1.  To install them, visit the Windows Update control panel and click on "check for updates."
  • You have a pre-release version of SP1 and need to uninstall it before installing SP1
  • You already have it.  To determine if you already have SP1 installed, Open the Start Menu, right click on Computer and left click on Properties.
  •  We released SP1 in these 5 languages:  English, French, Spanish, German, and Japanese.  If you have any other language installed, SP1 will not yet be offered to you.  (You might not even know if you have an additional language installed -- check the Regional and Language Options control panel to see which languages you have installed.)
  • Back in February we announced that we'd be using Windows Update to help make the update as seamless as possible for our users.  Windows Update will detect drivers that we know may be problematic when updating to SP1 and will not offer the service pack until an update has been installed.
Microsoft Knowledgebase Article:

Windows Vista Service Pack 1 is not available for installation from Windows Update and is not offered by Automatic Updates

Microsoft Windows Vista Team Blog Article:

Windows Vista SP1 Released to Windows Update