Windows Vista SP1
22 February 2008
It appears that Microsoft has released Service Pack 1 (SP1) early for Windows Vista x64 users - many are reporting that SP1 is officially available in their Windows Updates. It is currently unclear as to whether this release is accidental or not, but if you want to install SP1 this way keep in mind that there are three pre-requisite updates (KB938371, KB937287, and KB935509) which you should install before installing SP1 via Windows Update. Note that the standalone SP1 package doesn’t need these updates prior to installation.
Update: It appears this loophole may have been closed now.
Update 2: Microsoft has confirmed it was an accident.
source, site tweaksguide