If one installs Service Pack 1 (as opposed to installing a version of vista that included service pack one), copies of all the updated files are retained so that Servce Pack 1 can be uninstalled. Once you are satisfied that you won't need to remove service pack one, there is a utility that can remove thse obsolete files. Just find and run c:\Windows\System32\vsp1cln.exe. Note that the .exe extention is hidden by default in Windows Explorer.
I appear to have reclaimed about 2Gb of diskspace, which is a drop in the ocean if Vista is installed on a huge disk, but for those with small hard drives (often of the small and fast variety, notably WR (Veloci)Raptors and recent solid state drives) it's a noticeable space saving. Either way it's another game that can be squeezed in.
vsp1cln.exe is explained on Microsoft TechNet
